Penny worries that she's screwed things up with her friends after her night with Raj. Meanwhile Sheldon takes command of the paintball team.
A battle of wills between Sheldon and Penny leaves Amy in the middle. Leonard tries to spice up his long-distance relationship with Priya.
Leonard goes as Amy's date to a colleague's wedding. Howard tries to convince Bernadette to live with his mum after they get married.
Raj is upset that he's the only one of the four guys not in a relationship, so Penny sets out to find him the perfect date.
Sheldon's old nemesis Wil Wheaton returns and invites everyone to a housewarming party. Sheldon threatens to 'de-friend' Leonard if he goes.
Sheldon's mum visits and soon becomes a firm favourite with his friends, much to Sheldon's annoyance.
Leonard questions his feelings for Priya when he kisses a girl he meets in the comic book store.
Amy is heartbroken when she finds out that Penny and Bernadette have been shopping for bridesmaids dresses without her.
Leonard and Penny attempt to hang out without anybody else around, while Sheldon faces his fear of birds.
Stuart from the comic book shop asks Amy out on a date. In jealousy, Sheldon asks her to be his girlfriend.
Leonard confronts the man who bullied him in high school, while Penny realises she did her fair share of tormenting in the past.
When Sheldon is dismissive of Amy's career, Penny, Raj and Leonard teach him that sometimes it's a good idea to pretend to care about things
In the 100th episode of the US sitcom, Leonard and Penny agree to give their relationship another chance, but they soon start to argue again
Leonard and Penny put their relationship through a 'beta test' and report problems they have with each other as 'bugs', while Sheldon and Amy seek internet fame
After Sheldon keeps him awake all night, an irate Leonard decides that he needs to break the roommate agreement, which formally ends their friendship
Sheldon helps Amy in her biology lab after he is forced to use up two weeks of holiday, but he is bored by the tasks she gives him
Sheldon and Kripke argue over who should get Professor Rothman's vacant corner office. Amy buys Penny some art to celebrate their friendship.
Sheldon's well-organised world is turned upside down when he learns that the barber who has cut his hair for years is ill in hospital
Amy is upset when Sheldon breaks his promise to accompany her to her aunt's birthday party in order to join the guys for a video game marathon
Penny buys Leonard and Sheldon some Star Trek memorabilia but she is irked when they refuse to remove the toys from their packaging
When Stephen Hawking comes to lecture at the university, Howard is given the task of maintaining his wheelchair, making Sheldon green with envy
Leonard, Sheldon and Raj throw a bachelor party for Howard, during which some stories about his sexual history are accidentally revealed
When Nasa inform Howard that his space mission has been rescheduled to an earlier date, he tries to use the wedding as an excuse not to go
In the last episode of season five, Bernadette decides that she wants to get married to Howard before his Nasa space mission