Howard gets caught in an argument between his mum and Bernadette while at the International Space Station. Raj is feeling lonely so Sheldon invites him along to his second anniversary date with Amy.
Sheldon intervenes when he learns that Penny is thinking about breaking up with Leonard, but he's hampered by his inability to keep a secret. Meanwhile, in space, the astronauts are picking on Howard.
Amy is jealous when Sheldon hires an attractive female assistant named Alex. However, it turns out that Alex is more interested in Leonard, which makes Penny feel threatened.
Howard is disappointed when he doesn't receive a hero's welcome on his return from space. Will he ever get the recognition he deserves as an astronaut?
The gang celebrates Halloween with a fancy dress party at Stuart's comic book shop, but Sheldon and Amy can't agree about which costumes to wear
Sheldon is excited when Stephen Hawking (guest starring as himself) agrees to play Words with Friends with him online. But Hawking's not happy when Sheldon starts to dominate the game.
Sheldon fails to defend Amy in an argument with Wil Wheaton while filming Fun with Flags. Guest stars Star Trek: The Next Generation actors Wil Wheaton and LeVar Burton.
The guys notice that Sheldon disappears for 20 minutes right after lunch every day. Filled with curiosity, they attempt to follow him and work out what on earth he's been doing.
When non-driver Sheldon finds out that his unused parking space at the university has been reallocated to Howard, he feels undermined and the two fall out
Sheldon begrudgingly takes care of Amy when she is sick, but she loves the extra attention so much that she is still pretending to be ill long after her recovery is complete
Sheldon reveals painful memories of Christmas past when Leonard persuades him to join in with a seasonal game of Dungeons & Dragons. Raj joins Penny, Bernadette and Amy on a ladies' night out.
When Sheldon's assistant Alex starts flirting with Leonard, Sheldon is displeased and decides to have a word with her about it
The guys take a road trip to a comic book convention in Bakersfield and visit a location where several episodes of Star Trek were filmed. But Leonard's car is stolen, leaving them stranded.
Sheldon is forced to work on the same project as his nemesis, Barry Kripke. Howard and Raj spend a fortune on personalised action figures.
Sheldon and Leonard have a row when Sheldon gives away the ending of the Harry Potter book that Leonard is reading
Leonard tries to give Penny the perfect Valentine's Day treat by taking her to a nice restaurant with Howard and Bernadette. Meanwhile, Raj and Stuart throw a singles party at the comic book shop.
After a terrible date with Lucy, who he met at Stuart's singles party, Raj vows never to leave his flat or interact with another woman again. Leonard, Howard and Bernadette try to cheer him up.
When Leonard, Sheldon and Howard have to volunteer for a university committee, they join one that advances the cause of women in science and arrange to speak to schoolgirls about their careers
When Howard and Bernadette get Sheldon to organise their untidy closet, Sheldon finds an unopened letter from Howard's long-absent father, who abandoned his family years earlier
When a tenured position becomes available at the university, Leonard, Sheldon, Raj and Kripke are all keen to apply for it
Amy employs a series of exercises to cure Sheldon of his compulsion to complete every task without fail and seek closure for every incident
When Leonard and Sheldon find out that Professor Proton, the host of a TV science show they watched as children, can be booked for private parties, they hire him to come and entertain them
When the girls' planned trip to Las Vegas falls through, the guys invite them to a Dungeons & Dragons date night, which results in Sheldon and Amy's relationship taking an unexpected turn...
Leonard is offered an exciting job opportunity that will keep him out of the country for several months. Meanwhile, Raj pushes the boundaries of his relationship with Lucy.