The Big Bang Theory

Amy tries to help Sheldon get over his compulsive need for closure when one of his favorite TV shows is canceled; Raj discovers a secret about Lucy.

The guys fight over what message Raj should write for NASA in case of alien contact; Penny has to decide between auditioning for a movie or keeping her pharmaceutical sales job.

Wolowitz questions his engineering skills after he and Sheldon can't get a toy drone to fly; a canceled flight puts Leonard's plans in jeopardy; Raj sets his parents against one another after he's cut off financially.

Amy and Sheldon's relationship takes a surprising turn during a game of ``Dungeons & Dragons``; Lucy and Koothrappali have an awkward date.

The girls get involved when Leonard, Sheldon and Raj compete for tenure at the University.

When Leonard receives an exciting opportunity to work overseas, Penny and Sheldon are thrown for a loop.

While facing dramatic changes in his relationship with Amy, Sheldon pushes Penny and Leonard to choose a wedding date.

While Sheldon is in Texas, the rest of the gang realize how much he has changed their lives as they decorate the apartment Christmas tree.

Realizing he will not live long enough to download his consciousness into a robot body, Sheldon tries to extend his life span.

Leonard and Sheldon's friendship is tested; Wolowitz's relationship with his mom causes a threat to his masculinity.

Amy and Leonard try to help Sheldon deal with his shame; Raj stays with Bernadette and Howard for a week.

As Leonard tries to help Penny with a disappointment, Penny asks him a bold question about their relationship; Sheldon wants to learn how to be funny.

Sheldon and Penny share secrets while Leonard is away; Wolowitz helps Raj get over his ex.

When a mistake comes to light, Sheldon makes Leonard walk a mile in his shoes; Penny decides to confront Raj's ex.

Raj and Howard set up an online dating profile for Sheldon and find the perfect woman.

When Sheldon's and Leonard's mothers finally meet, personalities collide; Howard decides to do his share of the chores -- with a little help from his friends.

Sheldon's stage fright stands between him and a coveted award.

Sheldon invites a famous physicist to the apartment, and competes with Leonard for her attention.

When Professor Proton asks Leonard for help with an experiment, Sheldon retaliates by befriending Proton's rival, Bill Nye.

Sheldon wants revenge after Amy ruins one of his favorite films; Leonard finds a way to get Penny to do anything; Stuart and Raj make online dating profiles.

Penny tags along on Sheldon's first date; Wolowitz finds a new use for a robotic arm.

Sheldon's latest scientific breakthrough leaves him feeling like a fraud; a romantic gesture between Howard and Bernadette causes Penny to up her game with Leonard.

Sheldon goes to extremes to keep Leonard and Penny happy when they stop speaking to each other.

Leonard tells Penny about how he met Sheldon and what happened to the elevator.

When the gang decides to spend Thanksgiving at Mrs. Wolowitz's house, Sheldon protests; Leonard and Penny deal with an error from her past.

Penny and Leonard have a fight that threatens their relationship; Sheldon and Wil Wheaton face off once again.

Sheldon and Amy's relationship is tested when she takes a job at his university; a fight with Bernadette leaves Howard sleeping on Raj's couch.

Sheldon must learn how to relax when he is forced to take a vacation; Leonard tries to support Penny when she quits her job; Bernadette needs to replace one of Stuart's comic books.

When Sheldon searches for an alternative to human companionship, the guys become concerned.

A paintball game leads to a fight between Sheldon and Penny, and romance for Leslie and Wolowitz.

Sheldon's solution to Penny's financial difficulty leads to a confrontation between Leonard and Penny's ex-boyfriend.

Sheldon develops a scientific procedure for making friends; Kripke controls a new supercomputer.

An attractive comic book artist tests Leonard and Priya's relationship; Sheldon tries to scare his friends.

Amy is caught in the middle when Penny and Sheldon get into an argument over a pre-owned chair; Leonard tries to make his long-distance relationship more exciting.

Amy is devastated when Penny and Bernadette shop for wedding dresses without her.

Leonard and Penny are drawn together after a disastrous visit from Leonard's mother (Christine Baranski).

Leonard and Penny spend time alone; Sheldon tries to face his fear of birds.

Amy and Leonard try to become better acquainted; Bernadette, Howard and Mrs. Wolowitz spend the weekend together.

Sheldon finds himself competing with the gang for his mother's (Laurie Metcalf) attention when she visits.

Wil Wheaton invites Leonard and Sheldon to a party at his house; Howard receives an opportunity.

Leonard, Sheldon and Wolowitz are ecstatic to learn that a famous science-fiction actress (Summer Glau) is aboard the train they are taking to San Francisco.

Raj wants a girlfriend, so Penny introduces him to a woman he can communicate with.

Penny's home business turns into a complete nightmare when Sheldon takes over.

Penny is forced to rely on Sheldon when she is injured while Howard, Leonard and Raj are away.

Penny makes a surprising revelation about Leonard while on her date with Stuart; Wolowitz finds a critical design flaw in his NASA project.

Sheldon teaches Leonard how to understand football; a kite-fighting incident threatens Howard and Raj's friendship.

Sheldon seeks revenge on his nemesis, Wil Wheaton; Wolowitz wants Penny to set him up with one of her friends.

Penny develops a rivalry with a sexy new female neighbor who threatens her position as queen bee of the building.

After tampering with Sheldon's Arctic-expedition data, the guys follow him back to Texas, which threatens Leonard's chances for romance with Penny.

Howard and Raj hit a Goth nightclub to meet women; Sheldon tries chocolate-based behavior modification on Penny.

Sheldon tries to make peace when Leonard and Penny get into an argument.

Wolowitz and Leonard decide to check out a local bar on Ladies' Night.

Penny and Leonard try to recover from an awkward hookup; Howard and Sheldon make a high-stakes bet.

While Leonard prepares for a three-month science expedition, he and Penny re-evaluate their feelings for each other.

Raj faces deportation if he does not find a job; Leonard and Penny deal with Howard as a third wheel.

Koothrappali and Leonard take Wolowitz to Las Vegas; Sheldon is forced to bunk with Penny when he gets locked out of his apartment.

After their apartment is robbed, Leonard and Sheldon turn to their friends to create a state-of-the-art security system.

Sheldon tries to figure out the answer to a physics problem by working with Penny.

Raj and Sheldon try to simulate a dark-matter research expedition; after a fight about money, Penny and Leonard turn to Bernadette and Wolowitz for advice.

Wolowitz feels nervous when he is asked to throw the first pitch at a Los Angeles Angels game; Penny and Leonard want to prove Sheldon wrong after he says he and Amy are the better couple.

During their weekend in Las Vegas, Bernadette and Amy accuse Penny of being a buzz kill; the guys try to invent the next big thing.

Sheldon tries to help Penny understand Leonard's work; Wolowitz gets jealous when Leonard starts hanging out with his new girlfriend.

Raj's honesty comes back to haunt him when his new girlfriend is unfriendly to Penny; the guys think about becoming part owners in Stuart's store.

When Sheldon must teach a class, Howard decides to take it; Amy takes advantage of the tension between Penny and Bernadette.

Leonard must choose who to bring with him when he goes to see the Large Hadron Collider on Valentine's Day.

Leonard and Amy make an unexpected trip to Arizona to fetch Sheldon; Penny interviews for a job at Bernadette's company; Stuart's relationship with Mrs. Wolowitz disturbs Howard.

Leonard is horrified when his mother (Christine Baranski) visits.

The guys try to help when Penny is faced with an awkward dilemma at work; Bernadette and Amy discuss sexuality and female scientists.

Sheldon feels pressured to participate in romantic traditions when the gang re-creates a high-school prom on the roof.

Sheldon and Koothrappali attend a university mixer; Leonard discovers that Penny believes in psychics.

After being humiliated on the radio, Sheldon vows to destroy Kripke; Wolowitz tries to not ruin his new relationship with Bernadette.

While the guys camp out for a long-awaited movie screening, Penny and Bernadette invite Amy to her first slumber party.

Amy realizes she has feelings for Penny's ex; Koothrappali and Wolowitz try to prove who would be a better superhero.

Sheldon asks his friends to challenge him in hope of achieving a scientific breakthrough; Wolowitz invents a game about Raj.

Sheldon's interview with the FBI jeopardizes Wolowitz's security clearance.

A wealthy donor makes Leonard question what he is willing to do for science.

Penny asks Leonard to lie to her father (Keith Carradine); Howard, Raj and Bernadette pull an all-nighter at a telescope.

Leonard's idea for a smartphone application causes friction in his friendship with Sheldon.

Sheldon says goodbye to Fun With Flags; Howard, Leonard and Raj look through their dead professor's research; Bernadette learns her co-workers' true feelings.

Leonard and Wolowitz clash over a work emergency; Amy throws a Victorian Christmas party which Sheldon resents.

Wolowitz considers taking things with Bernadette to the next level; Leonard and Raj's sister rekindle their relationship.

While the gang waits for the reopening of Stuart's comic book store Wolowitz gets some shocking news; Penny has trouble taking her own advice; Leonard and Raj think they spot Nathan Fillion at a restaurant.

Leonard realizes he is the only one in his group without a girlfriend; Sheldon wants to get rid of his girlfriend.

Sheldon ends up in traffic court after getting a ticket because of Penny.

An embarrassing secret of Wolowitz's is revealed; Raj and Sheldon get into a war at work.

Penny and Sheldon design an experiment -- as a joke -- to make participants fall in love; Howard and Bernadette get into trouble at the airport.

Leonard keeps an affair a secret from Sheldon, Howard and Raj.

The guys try to bond with Penny's new boyfriend through a costume contest.

Bernadette runs into her ex-boyfriend (Rick Fox) while attending a science event with Wolowitz.

When the guys buy a ring from ``The Lord of the Rings`` at a garage sale, the item threatens to tear them apart.

Leonard has to choose between Penny and his new girlfriend, Priya; Howard's magic trick drives Sheldon crazy.

Leonard has minor surgery on his nose, causing Sheldon to overreact; Bernadette and Howard work on their marriage.

When the guys agree to do something the girls like, Sheldon and Leonard end up having a confrontation.

Sheldon takes acting lessons from Penny in hope of becoming a better teacher; Koothrappali fantasizes about his friend's girlfriend.

Sheldon is upset when he realizes Leonard is the center of their group of friends.

Sheldon and Leonard must defend themselves from an online bully's criticism after they co-publish a paper; Bernadette, Penny and Amy confront embarrassing moments during a girls night.

Sheldon and the gang are confused when a young grad student is attracted to him.

Leonard and his friends stage an intervention to force Sheldon to face his fear of driving.

Leonard inadvertently steals away the new woman Wolowitz thinks is the love of his life.

When Sheldon bans Penny from the apartment for a series of petty infractions, Penny vows to get revenge.

Sheldon is thrilled when Leonard starts dating a woman who meets Sheldon's stringent standards.

When Penny's comment about Wolowitz's love life sends him into a depressed stupor, his friends lose their most important teammate in an upcoming fighting-robot competition.

Leonard asks Penny for advice when his new relationship starts moving too fast.

Priya dissects the roommate agreement; the ladies take Sheldon dancing.

Howard's mom winds up in the hospital after hearing news of her son's engagement to Bernadette.

Amy and Sheldon spread gossip as part of an experiment; Howard decides to take a big step in his relationship with Bernadette.

Bernadette gets her Ph.D., inspiring the guys to tease Wolowitz about his lack of a doctorate; Raj becomes Sheldon's roommate.

Penny worries that she has ruined her relationship with her friends; Sheldon takes over the paintball team.

Raj tries to cure his social anxiety; Penny spies on Leonard and his girlfriend through Bernadette.

The guys try to figure out who hacked Sheldon's online-game account.

Unable to get Comic-Con tickets, Sheldon tries to hold his own convention and ends up spending a wild evening with James Earl Jones; the ladies try to act maturely.

An impending change to his living arrangements inspires Sheldon to revert to 2003, before he met Penny and Leonard; Raj and Howard form a band.

Leonard, Sheldon and Wolowitz file for a patent on their new invention; Penny, Amy and Bernadette give dating advice to Koothrappali after his breakup with Emily.

A flat tire during Leonard's bachelor party weekend in Mexico tests the guys' skills; Penny is compelled to tell her family that she and Leonard eloped.

Sheldon shares a secret about Leonard when the two go on a weekend trip with Penny and Amy; Koothrappali goes all out with pregnancy gifts.

For the first time in nine years, the gang celebrates Sheldon's birthday -- surprising him with a special guest.

The group struggles through an awkward morning after Sheldon's mother and Leonard's father spend an evening together; Penny's family arrives.

Leonard's recently divorced parents and Sheldon's devoutly religious mother visit; when they are contacted about their guidance system, Wolowitz and Koothrappali think the government is after them.

Sheldon has a song stuck in his head; despite their awkward date, Amy asks Dave over for dinner; Koothrappali and Wolowitz are obsessed with a fan of their band.

After dealing with Sheldon while he was sick, Leonard, Penny and the gang attempt to treat themselves to a Sheldon-free weekend.

The gang runs into Penny's old boyfriend while out at a wine bar; Bernadette spends an evening with Sheldon; Koothrappali's introduction of his new girlfriend doesn't go as planned.

Leonard and Sheldon go to great lengths to secure the helium they need during a nation-wide shortage; Bernadette and Penny try to find Amy a new man on a dating app.

Sheldon wants to go to a movie screening, so he pays Stuart to spend the day with Amy; Penny has a difficult time connecting with Leonard's visiting mother.

Sheldon's excitement about his Meemaw's visit fades when she butts heads with Amy; Raj questions his relationship with Emily after meeting someone new.

Sheldon, Wolowitz and Koothrappali post an ad online to find a new girlfriend for Sheldon; Penny and Bernadette decide to spy when Amy is secretive about who she is dating.

After dating for more than five years, Amy and Sheldon spend the night together for the first time; Leonard, Wolowitz, and Koothrappali must decide what to do with an extra movie ticket.

Barry Kripke expresses interest in Amy while leading a fencing lesson for the guys; when Stuart tries to attract female customers to the store, Penny, Amy and Bernadette help out.

Amy and Sheldon spend Thanksgiving at the aquarium -- as friends; Wolowitz volunteers at the soup kitchen with Raj, Emily and Bernadette; Penny realizes that Leonard knows her better than she thought.

The gang celebrates news of Bernadette's pregnancy by singing karaoke; Wolowitz's reaction to the pregnancy announcement worries Bernadette; Wolowitz believes he needs to make more money to support his growing family.

Leonard agrees to meet with a psychiatrist (Jane Kaczmarek) on Penny's behalf; Sheldon and Koothrappali work together on an astronomical discovery; Wolowitz and Bernadette struggle with their feelings when Stuart moves out.

Leonard tries to ease Penny's mind; Sheldon plans a special episode of ``Fun With Flags``; Bernadette feels bad about keeping a secret.

Sheldon makes a surprise revelation when Amy buys him a new laptop; Leonard and Wolowitz get sold out by Koothrappali after lying to their wives to attend a movie screening.

Sheldon struggles to keep his emotions in check when he is interviewed for a documentary about Spock; Penny and Leonard make a surprising discovery; Wolowitz and Bernadette get into a disagreement about remodeling his childhood home.

Amy and Sheldon host a Valentine's Day edition of Fun With Flags; Penny and Leonard wrestle with getting older; Howard and Bernadette make a discovery in their hot tub; Raj is torn between Emily and another woman.

During a group get-together, a minor argument between Sheldon and Leonard turns into a heated debate, leaving the rest of the gang to choose sides.

Sheldon and Amy have their first fight since moving in together; Koothrappali is shocked at the realities of financial independence when he stops receiving support from his father.

While Howard and Bernadette welcome their new baby into the world, Amy celebrates her own birthday with Sheldon.

Sheldon is determined to convince Amy they should procreate; Koothrappali is reluctant to tell the gang what the new woman he's dating does for a living.

Leonard is less than thrilled when Penny considers accepting a job offer from Zack, her ex-boyfriend.

After her apartment floods, Amy suggests that she and Sheldon move in together; Koothrappali finds out the gender of Howard and Bernadette's baby before they do.

Leonard, Penny and Raj settle into their new living arrangement; Sheldon expresses interest in Amy's work.

Cut off from his father's fortune, Koothrappali appoints Sheldon to help him get out of debt; Penny considers joining Leonard at Comic-Con.

Sheldon tries to stay awake with an energy drink as the gang races to finish its government project on time; Penny and Amy attend a gathering at Bert the Geologist's house; Koothrappali learns Bernadette's true feeling about her pregnancy.

Sheldon tests an MIT invention after hearing it can help him read people's emotions; Leonard finds out Penny's brother is coming to town; Raj asks his ex-girlfriends to give him dating advice.

Leonard and Penny offer Sheldon's old room to Raj after he moves out of his apartment.

Penny learns there are fans of her performance in ``Serial Ape-ist`` when she and Leonard attend a Comic Con event; Amy hosts a brunch at Penny's apartment; Wolowitz makes an impulsive purchase.

Sheldon gets jealous when a co-worker wins an award; Wolowitz finds an old remote control Stephen Hawking action figure that he made.

After celebrating the completion of the top-secret Air Force project, Leonard, Sheldon and Wolowitz get an unwelcome surprise; Amy receives an offer for a summer position as a visiting researcher.

Sheldon and Amy visit Penny and Leonard after the holidays and tell them about their terrible trip to Texas.

Things do not run smoothly when Amy moves in with Sheldon, and Leonard and Penny have to intervene; Howard and Bernadette find unexpected guests at their house when they decide to cancel their vacation plans.

Leonard and Wolowitz try to distract Sheldon when he slows the progress of their guidance system; Raj and Stuart look after baby Halley while Penny and Amy take Bernadette for a night out.

The gang is concerned when Sheldon's former admirer, Dr. Ramona Nowitzki, resurfaces while Amy is away at Princeton.

Wolowitz has a meeting with Colonel Williams, a representative from the Air Force Department of Material Command.

Sheldon and Leonard disagree about how to divide their shared belongings; Koothrappali and Stuart fight to be the most helpful to Bernadette during her final weeks of pregnancy.

Amy answers Sheldon's marriage proposal; Howard and Bernadette struggle with unexpected news.

Sheldon pushes himself to the limit when he collaborates on projects with Leonard and Wolowitz as well as Amy on the same day; Bernadette nears the end of her maternity leave and must return to work.

Leonard's job is at stake after he gives an embarrassing radio interview; Amy and Bernadette bond over having to hide their success from Sheldon and Howard.

Penny thinks Leonard is taking her for granted and decides to go on a spa weekend with Amy; Wolowitz and Koothrappali try to fix a squeaky floorboard in the baby's room.

Howard and Bernadette struggle to leave Halley in day care; Bert introduces the gang to his new girlfriend, Rebecca.

Amy lies about repairs being made to her apartment so she can continue living with Sheldon; Leonard lets Penny decorate Sheldon's old room.

Leonard jumps through hoops to help secure the perfect wedding venue for Sheldon and Amy; Bernadette and Wolowitz have a hard time deciding who should stay at home with the kids and who should go back to work.

Sheldon tries to teach the guys a lesson after they cut him out of a potentially valuable investment; a seven-year-old video reveals a secret about Leonard and Penny's relationship.

When Amy's parents and Sheldon's family arrive, everybody is focused on making sure the wedding arrangements go according to plan -- everyone except the bride and groom.

Sheldon plans an authentic ``Little House on the Prairie`` dinner for Amy's birthday; Leonard receives a Christmas letter from his brother.

Sheldon and Amy are devastated after learning from a Russian paper that Super Asymmetry has already been discovered and disproved; Bernadette wants to beat Howard in a popular video game.

Sheldon and Koothrappali confide in Bernadette when they can't handle Amy and Wolowitz working together; Penny and Leonard learn a secret from an unlikely source about dealing with Sheldon.

Sheldon's comic book store experience changes when writer Neil Gaiman puts Stuart's store on the map; friendships are threatened when Koothrappali takes credit for Penny's astronomical discovery.

When Penny and Bernadette go to San Diego for a pharmaceutical sales convention, Wolowitz is left in charge of the kids; Sheldon finds a book about experimenting on kids.

Sheldon and Amy try to use math to eliminate wedding planning stress; Koothrappali ''breaks up'' with Wolowitz after realizing he is hurting his confidence.

Sheldon and Amy are thrilled when their super asymmetry theory is proven by two physicists, until they try to kick Amy off the Nobel nomination; Bernadette has a big success at work.

Sheldon and Amy's honeymoon runs aground in New York; Penny and Leonard discover they are uncomfortably similar to Amy's parents; Koothrappali insults physicist Neil deGrasse Tyson and starts a Twitter war.

Sheldon tries to bond with Amy's father, but when Wolowitz lures Mr. Fowler away with magic tricks, Sheldon has no choice but to bond with Mrs. Fowler; Koothrappali and Anu try to get physical for the first time.

The gang uses deception and betrayal to make it to the one open seat at Wil Wheaton's celebrity Dungeons and Dragons game with William Shatner, Joe Manganiello, Kareem Abdul-Jabbar and Kevin Smith.

Penny tries to throw a wrench into Leonard's plan to be a sperm donor for her ex-boyfriend; Wolowitz, Bernadette, Anu and Koothrappali turn Koothrappali's canceled bachelor party into a couple's trip aboard the ``vomit comet.``

Wolowitz and Sheldon bond over having disappointing fathers; Leonard is disturbed when his mother finds a new best friend in Penny.

Penny gets the opportunity to host Bill Gates at work when his company wants to partner with hers; Leonard, Koothrappali and Wolowitz do everything in their power to meet him; Sheldon thinks he is the victim of a prank.

Sheldon tries to keep his collaboration with Bert a secret; Penny and Bernadette coach Koothrappali on navigating his relationship with Ruchi.

Leonard is caught between a rock and a hard place when he becomes responsible for distributing extra grant money; Bernadette turns the backyard playhouse into a hideaway to get away from her husband and kids.

When Wolowitz dresses up as Sheldon for Halloween, Sheldon seeks retaliation at Leonard and Penny's Halloween party; Leonard is shocked when Penny doesn't remember their first kiss.

When competitors Pemberton and Campbell charm America on a publicity tour, Sheldon and Amy try to win over Nobel laureates Kip Thorne, George Smoot and Frances Arnold; Halley's fear of the dark leads to opportunity and conflict.

Sheldon and Amy subject their friends to a series of secret experiments to discover which two would be most qualified to be the best man and the maid of honor at their wedding; Penny reveals her true feelings about Amy.

Sheldon is thrilled to help Bernadette and Wolowitz navigate bureaucratic paperwork, until he discovers they are breaking the law; Leonard is disappointed when his friends exclude him from a scientific project.

Sheldon goes to Las Vegas to win money for science; Penny and Bernadette take Amy wedding dress shopping, but her terrible choice entangles them in a web of lies.

When Bernadette won't go into labor, her friends try different tactics to get things started; Wolowitz confronts Bernadette after Amy accidentally reveals she's already chosen their son's name.

Sheldon petitions Wil Wheaton to appear on the new ``Professor Proton`` show, but Wil offers a role to Amy instead; Penny gets angry when she thinks the female protagonist in Leonard's novel is based on her.

Penny and Leonard organize a paintball game, which results in mayhem when Sheldon becomes jealous of Amy; Koothrappali catches Anu with her ex-boyfriend; Stuart doesn't want to move in with Denise.

When Amy doesn't have time to work on super-asymmetry, Sheldon goes to great lengths to ensure she's available; Koothrappali doesn't want Wolowitz to join him in hosting a show at the planetarium.

Life gets complicated when Stuart starts bringing his new girlfriend home; Penny and Leonard talk about starting a family; Koothrappali explores an arranged marriage.

Penny's ex-boyfriend Zack learns that he is infertile, so he asks Leonard for help; Koothrappali attempts to mend things with his ex-fiancee Anu.

Sheldon auditions for the role of the new Professor Proton; Penny takes care of Halley when Bernadette and Wolowitz need bed rest.

A brilliant but reclusive scientist, Doctor Wolcott, invites Sheldon to his cabin in the middle of nowhere; Penny and Bernadette improvise after Amy doesn't like the tame bachelorette party they planned for her.

As Sheldon stresses about picking a wedding date, Amy tries to convince him he has a more laid-back side; Koothrappali and Stuart compete to win the heart of Bernadette's new co-worker.

Koothrappali learns the woman he's dating has a very upset husband; Sheldon rents his old room for a quiet place to work.

Sheldon's mother won't attend his wedding unless he invites his brother; the Wolowitz kids inadvertently infest Amy, Bernadette, Wolowitz and Koothrappali with pink eye.

Sheldon kicks Amy out to work solo; Amy and Leonard bond during a series of science experiments; Bert replaces Wolowitz in the band; Sheldon finds Penny a surprising source of scientific inspiration.

When Sheldon won't tell anyone why he's never mentioned Tam, his childhood best friend, Leonard takes matters into his own hands; Bernadette and Penny take Raj's fiancee out to dinner to get to know her.

Leonard decides to run against Sheldon for president of the tenants association; Bernadette encourages Wolowitz and Koothrappali to find the owner of a drone after they find it in the backyard.

Leonard and Wolowitz are furious when they learn Sheldon worked with the military behind their backs; Bernadette suspects Ruchi is trying to steal her job.

A VHS tape from Sheldon's past inspires him not to give up; Bernadette turns into a pageant mom when she tries to help Wolowitz with his Magic Castle audition.

Sheldon and Amy go crazy trying to figure out what ``perfect gift`` Leonard and Penny gave them for their wedding; Koothrappali decides to settle down and asks his father to arrange a marriage for him.

Leonard is happy to meet new neighbor Penny, but his roommate thinks he is chasing an impossible dream.

Penny's first date with Leonard goes awry; Penny finds Sheldon to be an unwilling confidant.

Sheldon creates a monster when he introduces Penny to online gaming.

When Sheldon tries to prove his mental superiority as they train for the Physics Bowl, the guys kick him off the team and bring his nemesis (Sara Gilbert) aboard.

Leonard volunteers to sign for a package, hoping to make a good impression on Penny.

Sheldon and Amy await big news.

After seeing Penny with a handsome new guy, Leonard rushes into a rebound relationship with Sheldon's nemesis, Leslie Winkle.

Penny tries to mediate when Sheldon and Leonard disagree on whether to present their joint findings at a physics conference.

Koothrappali is worried people won't take him seriously in his own field after publishing a paper that suggests he may have discovered alien life; Leonard wants to be the principal investigator on a plasma physics study.

Penny sleeps on Leonard and Sheldon's couch when Wolowitz seduces her friend and takes over her apartment; in need of a fourth member, the guys ask Penny to join their ``Halo`` team.

Unhappy to learn that Penny is seeing someone, Leonard summons the courage to ask another woman on a date.

On a blind date, Koothrappali discovers that he can talk to the opposite sex after a few drinks, but his date takes an interest in Sheldon.

Leonard, realizing his pursuit of Penny is fruitless, has an unexpected encounter with a fellow scientist (Sara Gilbert).

The weight of winning a Nobel Prize weighs heavy on Amy's shoulders; Koothrappali and Wolowitz try to relive the good old days after Wolowitz buys a scooter that looks like the one he had years ago.

Sheldon and Leonard look for a way to derail a 15-year-old physics prodigy's research.

Leonard and Sheldon try to avoid seeing Penny perform after hearing her sing, but the elaborate lie Sheldon tells leads to an unexpected result.

Getting fired gives Sheldon the opportunity to explore life beyond physics, but when he spends his time weaving ponchos and acting like a shut-in, Leonard calls Sheldon's mother for assistance.

Leonard is pleasantly surprised when Beverly comes to visit, and she's genuinely kind to him, until he finds out the real reason she's there; Denise and Stuart overstay their welcome at the Wolowitz house.

Leonard, Sheldon, Wolowitz and Koothrappali attend Penny's Halloween party.

Leonard and the guys buy a time machine prop from a classic movie; Penny tells the guys their obsession with comics and toys is pathetic.

Leonard and the boys avoid Sheldon when he gets sick, leaving Penny to nurse him back to health.

Penny learns that Leonard does not celebrate birthdays, so she plans a surprise party; Sheldon unexpectedly gets to live out one of his biggest fantasies in an electronics store.

Kripke has proof that Dr. Pemberton plagiarized his thesis in college, and Sheldon and Amy aren't sure if they should turn him in or not; Wolowitz learns that Bernadette wasn't the only waitress at the Cheesecake Factory who had a crush on him.

Leonard devises a plan to eliminate his rivals when he, Wolowitz and Koothrappali all fall for Sheldon's twin sister, but his plan backfires when Sheldon decides to choose who his sister dates.

Bernadette and Wolowitz leave their kids for the first time; Penny and Leonard try to keep a secret; Sheldon and Amy stick together; Koothrappali makes a new friend; the gang travels together into an uncharted future.

When Leonard finally works up the courage to ask Penny out on a date, they each turn to Sheldon for advice.

Penny starts dating Leonard's handsome colleague; Leonard and his friends are tortured by Sheldon's obsession with gift-giving etiquette.

Raj gets an inflated ego and makes a play for Penny after being referenced in a magazine article.

Raj designs a scavenger hunt that brings out the best -- and the worst -- in the whole gang.

As the guys don costumes and go on a road trip to a comic book convention, the ladies decide to stay home and try to interpret a comic book.

Sheldon is jealous when Stuart (Kevin Sussman), a guy from the comic book store, asks Amy out on a date.

The gang heads to Stuart's comic book store to celebrate Halloween; Leonard uses science to seduce Penny.

When Penny buys Leonard and Sheldon ``Star Trek'' collectibles, Sheldon finds himself haunted by Mr. Spock; after a blind date, Raj believes he has met his future wife.

A game threatens Sheldon's relationship with Stephen Hawking; Penny secretly signs up for a college class.

When Wolowitz works with Stephen Hawking, Sheldon is determined to meet the famous scientist.

Amy feels threatened by Sheldon's new assistant; Wolowitz starts to feel the pressures of being in space.

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