With Wolowitz AWOL with one of her friends, Penny's recruited to fill in for him on the guys' Halo team

With his friends knowing how sour he acts when sick—and avoiding him as a result—Sheldon is nursed by Penny, who hasn't a clue what she's in for.

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

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

Penny's criticism of Wolowitz's romantic life pushes him into depression and AWOL for a fighting robot battle the rest of guys are depending on him for.

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

Penny squares off with a new beautiful female neighbor who may become the building's "new Penny."

The guys' plan to work over the summer at the North Pole makes Penny revaluate her feelings for Leonard.

A squabble between Leonard and Penny sees Sheldon playing mediator, though out of his own incovenience from the sitiation rather than altruism.

After he's humiliated on National Public Radio, Sheldon vows to destroy Kripke, while Wolowitz tries not to destroy his new relationship with Bernadette.

Penny gets help from Sheldon in order to understand what Leonard's work is, while Wolowitz develops a jealousy when Leonard starts hanging out with his new girlfriend.

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

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

An embarrassing secret of Wolowitz's comes to light, while Sheldon and Raj have a small war at work.

Leonard realizes he’s the only one without a girlfriend, while Sheldon wants to get rid of his.

As Wolowitz contemplates taking things with Bernadette to the next level, Leonard rekindles his relationship with Raj's sister.

Sheldon's online-game account is hacked by Todd Zarnecki, who then proceeds to steal all his items prompting the guys to go on a quest to retrieve them. Penny, Bernadette and Amy discuss how Penny groomed Leonard to be willing to try various things and more adventurous boyfriend... for some other woman — Priya.

When Howard finally breaks the news to his mom that he's engaged to Bernadette, it lands her in the hospital.

When Bernadette receives her PhD, Wolowitz feels emasculated. Raj and Leonard swap places and Penny does something she's regrets.

Leonard and Amy get to know each other better, while Howard and Bernadette spend the weekend with Mrs. Wolowitz.

Sheldon and Leonard get invited to a party at Wil Wheaton's house, and Howard gets an out-of-this-world opportunity.

Sheldon gets in trouble with Amy when he isn't impressed by her recent accomplishment, and Howard must come to grips with Bernadette's dislike of children.

Sheldon's selfish demands force Leonard to reconsider their friendship. Meanwhile, Wolowitz tries to pick his astronaut nickname.

Sheldon becomes agitated when his regular barber is out sick. Astronaut training causes Howard to reconsider wanting to go into space. Penny beats Leonard in chess, the first time she plays.

Sheldon must choose between joining Amy at her aunt's birthday party or playing videogames all weekend with the guys.

When Wolowitz gets to work with Stephen Hawking, Sheldon is willing to do anything to meet his hero.

The fifth season ends with Howard and Bernadette deciding to get married before his NASA mission, causing the gang to scurry into overdrive in an effort to stage the wedding. NASA astronaut Mike Massimino has a cameo as himself.

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

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.

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

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.

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.

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.

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.

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.

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.

The girls decide to re-enact their high school proms with the guys. While Sheldon feels under pressure to commit to his relationship with Amy, Howard worries about who Stuart will bring as his date.

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