Leonard's attempts to woo Penny are constantly thwarted, while Sheldon heads to Texas after learning that his friends meddled with his expedition data
Leonard and Penny feel awkward around each other after their romantic liaison, while Sheldon and Howard take bets to determine a species of cricket
Sheldon rewards Penny with chocolate to modify her behaviour, while Howard and Raj go to a goth nightclub
Raj reluctantly accepts a job offer from Sheldon so that he won't be sent back to India. Leonard and Penny try to get some time for themselves.
Sheldon enters a competition that pits him against Star Trek actor Wil Wheaton, whom Sheldon considers his arch-enemy after he snubbed him as a child at a Star Trek convention
Sheldon teaches Leonard about football so that he can fit in with Penny's friends, but Leonard alienates the group with his new knowledge
Sheldon is upset when Leonard and Penny argue about Penny's male friend staying over and he has to act as peacemaker
Leonard, Raj and Howard go on a camping trip that alters their minds. Sheldon has to come to Penny's rescue after she dislocates her shoulder when she slips in the shower.
Howard is scared about committing to Bernadette so he avoids her. Then he realises he's made a mistake and, to compensate, he proposes to her.
Penny feels left out when Bernadette asks Sheldon to teach her about science. And Howard is jealous when Bernadette asks Leonard about his work.
Leonard is horrified when his mother turns up unannounced, and then he finds out that Sheldon knew about the visit
Sheldon and Raj go to a university party, Leonard disapproves of Penny's belief in psychics, and Raj lusts after a girl called Abby
Leonard and Sheldon's flat is burgled. Determined to outwit any thieves, the gang work on creating a state-of-the-art security system to protect the apartment. Can they deter the burglars?
Sheldon is baffled by a formula he's working on, so he gets a less mentally taxing job at the Cheesecake Factory to kick-start his creativity
Sheldon is gutted when Leonard asks Penny to go to Switzerland to see the world's largest particle accelerator instead of him
Sheldon is summoned to court for driving through a red light. He misses the chance to meet Stan Lee and riles the judge. Will he go to prison?
The gang squabble over a ring that they believe was used in The Lord of the Rings movies, while Sheldon seems to be turning into Gollum
Sheldon earns a prestigious prize but is scared of collecting the award at a ceremony because he knows he'll have to make a speech
Leonard tells Penny that he loves her. The gang go bowling and find that Sheldon's enemy Wil Wheaton is playing in the opposing team.
Sheldon hides his friendship with Penny from Leonard but his cover is soon blown. Can he remain friends with both Penny and Leonard?
Sheldon meets a famous physicist online and invites her over to the flat, where she spends her time flirting with Leonard
This episode flashes back to the start of Sheldon and Leonard's friendship seven years previously and explains the housemate agreement
Raj registers Sheldon for online dating and finds the perfect match: the female version of Sheldon. Penny seduces Leonard while she's drunk.