With his friends on dates, Raj must face his loneliness. Also, Wolowitz is caught in an argument between Bernadette and his mom, even while he’s in space.

When Sheldon learns that Penny is thinking about breaking up with Leonard, he tries to intervene. Meanwhile, in space, the other astronauts pick on Howard.

Amy feels threatened when Sheldon hires a young female assistant, and the pressure of being in space takes its toll on Wolowitz.

When Wolowitz returns from space, he doesn’t get the hero’s welcome he expected. Meanwhile, Game Night turns into a battle of the sexes.

The gang celebrates Halloween at Stuart’s comic book store. Meanwhile, Leonard seduces Penny with science.

Sheldon’s relationship with Stephen Hawking is threatened over a game of “Words with Friends,” and Penny secretly enrolls in a class at the local college.

Wolowitz struggles with moving out of his mother’s house. Meanwhile, Sheldon is caught in the middle of an argument between Amy and Wil Wheaton.

Raj and Howard try to to figure out why Sheldon disappears at 2:45 every afternoon; jealousy arises in Penny and Leonard's relationship.

When the University reassigns Sheldon’s parking spot to Wolowitz, their fight affects the whole gang.

When Amy gets sick, Sheldon tries to nurse her back to health; Wolowitz and his father-in-law plan a fishing trip.

During a Dungeons and Dragons game, Sheldon revisits Christmas memories; Amy, Penny and Bernadette look for a date for Raj.

When Sheldon is accused of sexual harassment, he ends up getting Raj, Leonard and Howard in trouble.

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 faces a crisis of confidence when he is forced to work with his nemesis, Barry Kripke; Raj and Howard buy expensive action figures of themselves.

A fight between Sheldon and Leonard impacts Amy and Penny's living arrangements; while Howard is away, Raj helps Mrs. Wolowitz out.

The guys search for a perfect Valentine's Day gift for their significant others; Stuart and Raj host a "lonely people" party at the comic book store.

After a bad date, Koothrappali decides to never leave his apartment; Penny's acting skills impress Sheldon.

Leonard, Sheldon and Howard give lectures on science to junior high students. The girls ditch work to go to Disneyland. Raj devises a date for Lucy that fixes their social anxiety problems.

Howard considers whether or not he should open a letter from his father; Penny and Leonard host a "grown-up" cocktail party.

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

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.

Leonard and Sheldon hire the host (Bob Newhart) of their favorite childhood TV show to perform; Bernadette and Howard run into trouble while dog-sitting for Raj's pet.

A date between Raj and Lucy takes both of them out of their comfort zones. After a trip to Las Vegas goes awry, the girls join in on a Dungeons and Dragons game, which causes Amy and Sheldon to re-evaluate their relationship.

When Leonard receives an exciting opportunity to work overseas, Penny and Sheldon are thrown for a loop. Raj and Lucy's relationship comes to a crossroad.

With Leonard away, Sheldon and Penny spend quality time together sharing intimate secrets. Also, Wolowitz helps Raj get over his ex-girlfriend.

Leonard and Sheldon's friendship is tested; Howard's latest foray into helping his mother has some disastrous side effects.

When the entire gang competes in a scavenger hunt designed by Raj, it brings out the best - and worst - in everyone.

Sheldon gets mad at Amy for ruining one of his favorite movies, Raj and Stuart try to make their online dating profiles more appealing, and Leonard tries to play the sympathy card with Penny.

Amy starts working at Caltech which causes friction with Sheldon. Howard agrees with Sheldon who mentions this to Bernadette causing a big fight for the Wolowitzes.

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 seeks payback when Arthur "Professor Proton" Jeffries asks Leonard for help on a project instead of him. Raj gets upset at Howard for crashing his night with the girls.

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

Sheldon protests when the gang drags him to Mrs. Wolowitz’s house for Thanksgiving. Meanwhile, Penny and Leonard must deal with a mistake from her past.

Leonard has to deal with the fallout of disproving Sheldon's new scientific discovery. Raj spends a week at Howard and Bernadette's apartment.

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

When Penny’s big acting break on NCIS is a bust, Leonard struggles to help her, which results in Penny asking Leonard a bold relationship question. Meanwhile, Raj and Stuart try to practice their social skills, and Sheldon navigates the intricacies of being funny.

Sheldon tries to relax after he is forced to take a vacation. Meanwhile, Leonard struggles to be supportive of Penny after she quits her job, and Bernadette seeks Stuart’s help in replacing one of Howard’s comic books. Also, Amy receives an unwanted attention from a co-worker.

After the guys can’t get Comic-Con tickets, Sheldon tries to hold his own convention and winds up spending a wild night with James Earl Jones. Meanwhile, the girls try to feel more mature by going to a fancy hotel for tea.

Sheldon and Amy take a train ride to Napa Valley with Howard and Bernadette on Valentine's Day. Leonard and Penny take Raj's dog to the vet when she accidentally eats some chocolate.

Leonard buys a dining room table and it causes Sheldon to reevaluate the changes in his life. Meanwhile, Wolowitz is offered a chance to go back to space and Bernadette struggles with whether or not to encourage him.

After arguing with Sheldon, Howard tries to make amends by taking him to NASA headquarters in Houston. Penny has second thoughts about quitting her waitressing job when her car dies. Amy gets involved in trying to find Raj a date.

When Sheldon visits his Mom in Houston, he is forced to confront a new reality. Meanwhile, Raj’s ‘Murder Mystery’ party starts some fights within the gang.

While Raj is wracked with guilt when he tries to date two women at once, Penny wrestles with whether to take a role in a cheesy movie and Sheldon is torn trying to choose between two gaming systems.

Sheldon faces a personal crisis after deciding he’s wasting his time with string theory. Meanwhile, a double date between Raj, Emily and the Wolowitzes ends in embarrassment for Howard.

To prepare for a date with Emily, Howard acclimates Raj to horror movies. Penny and Sheldon are upset when they discover Amy and Bernadette lied to avoid hanging out with them.

As the gang celebrates Star Wars Day, Sheldon has difficulty dealing with the recent death of Arthur "Professor Proton" Jeffries.

A bad day causes Penny to reevaluate her life choices – including Leonard. Meanwhile, Howard and Bernadette struggle to care for Mrs. Wolowitz, and Raj gets relationship advice from Sheldon.

Sheldon get upset about his career, the destruction of the comic book store and Leonard and Penny's future living arrangements. Howard and Bernadette can't keep hold of a caretaker for his mother. With his entire world changing around him, Sheldon considers a major move.

After Sheldon’s mother and Leonard's father share an evening together, everyone deals with an awkward morning the next day. Also, Penny’s family arrives for the wedding ceremony, including her anxiety-ridden mother, Susan, and her drug dealing brother, Randall.

Colonel Williams, an Air Force representative from the Department of Material Command sits down to talk with Wolowitz.

Tensions rise when the boys struggle to complete their government project on time and Sheldon tries an energy drink to stay awake. Also, Penny and Amy go to a “party” at Bert the geologist’s house, and Koothrappali learns Bernadette’s true feelings about her pregnancy when they clean out the future baby room.

When Amy’s apartment floods, she proposes a “cohabitation experiment” with Sheldon. Also, Howard and Bernadette are upset when Koothrappali learns the gender of the baby before they do.

Leonard and Penny must separate a quarreling Sheldon and Amy when their cohabitation does not go as planned, and Sheldon threatens to break off their relationship when Amy refuses to adhere to the bathroom schedule. Also, Howard and Bernadette find unexpected guests at their house when they decide to stay home from a planned vacation.

Penny is surprised to learn she has fans of her performance in "Serial Ape-ist" when she attends Van Nuys Comic Con with Leonard. Also, Amy hosts a Sheldon-style brunch at Penny’s apartment, and Wolowitz makes an impulsive purchase when he starts stressing out about the baby on the way.

In order to continue living with Sheldon, Amy lies to him about repairs being made to her apartment and they host an "on location" episode of "Fun With Flags" in Penny’s apartment. Also, Leonard lets Penny decorate Sheldon’s old room after he learns that she has been secretly moving Leonard’s collectibles into storage.

After a successful experiment combining their genes, Sheldon will stop at nothing to convince Amy they should procreate. Also, Koothrappali is embarrassed to tell the gang what the new woman he’s dating does for a living.

When Bert, a Caltech geologist, wins the MacArthur Genius fellowship, Sheldon is overcome with jealousy. Also, Wolowitz finds an old remote control Stephen Hawking action figure he invented.

Sheldon and Leonard try to divvy up their shared belongings, but can’t agree on anything. Also, Koothrappali and Stuart fight to be the most helpful during Bernadette’s final weeks of pregnancy.

Howard and Bernadette welcome their new baby and Sheldon and Amy celebrate Amy’s birthday.

Sheldon and Amy visit Leonard and Penny for the first time since the holidays and share details about their terrible trip to Texas.

When Penny feels that Leonard is taking her for granted, she decides to go on a spa weekend with Amy. Also, Wolowitz and Koothrappali try to fix a squeaky floorboard in the baby’s room.

Sheldon beta-tests an MIT invention after hearing it can help him read people’s emotions. Also, Leonard can’t hide his feelings when Penny announces her brother is coming to town, and Raj invites his ex-girlfriends over to give him dating advice.

Leonard and Wolowitz try to distract Sheldon when he slows the progress of their guidance system. Also, Penny and Amy take Bernadette out for the night, leaving Raj and Stuart to care for baby Halley.

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

With Koothrappali cut off from his father’s fortune, he appoints Sheldon to help him get out of debt. Also, Penny considers joining Leonard at Comic-Con.

When Raj moves out of his apartment, Leonard and Penny offer him Sheldon's old room.

Leonard, Penny and Raj adjust to their new living arrangement, and Sheldon takes an interest in Amy’s work.

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

Howard and Bernadette have a hard time leaving Halley in day care when Bernadette returns to work. Also, Bert introduces the gang to his new girlfriend, Rebecca.

Leonard isn’t thrilled when Penny considers taking a new job offered by her ex-boyfriend, Zack.

After Leonard, Sheldon and Wolowitz celebrate the completion of the top secret air force project, they are met with an unpleasant surprise. Also, Amy is offered a summer position as a visiting researcher.

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