Penny tags along on Sheldon's first ever date, with a woman he's been texting for months. Meanwhile, Howard finds a new use for a robotic arm.
Sheldon realises he won't live long enough to download his consciousness into a robot body, so he tries to extend his lifespan
The guys are concerned when a heartbroken Sheldon starts bulk-buying cats after he breaks up with Amy. Leonard realises he must get Sheldon's mother to visit.
An embarrassing secret of Howard's comes to light as he tries to win back Bernadette. Meanwhile, Sheldon and Raj engage in a battle of wills.
Leonard realises that he's the only one of the gang without a girlfriend, while Sheldon wants to get rid of his when she hatches a plan for him to meet her mother.
A visit from Raj's sister Priya creates tension among the guys - especially when Leonard coaxes Sheldon into hiding his romantic past with Priya from the rest of the gang.
Leonard believes his problem with women stems from a lack of confidence, and vows to be more self-assured around attractive women. Meanwhile, Howard has a call from the FBI.
Sheldon's movie-going plans are scuppered by his nemesis Wil Wheaton. Penny and Bernadette invite Amy to her first slumber party.
When Penny's father comes to visit, Leonard discovers that her parents still think that he and Penny are a couple
Amy discovers that she has feelings for Penny's ex-boyfriend Zack. Meanwhile, Raj and Howard try to prove who would be the better superhero.
The guys warm to Penny's new boyfriend Zack and find a way to bond with him.
Leonard's idea for a smartphone application derails his friendship with Sheldon as they fight for control of the project
Howard feels threatened when Bernadette runs into her hulk of an ex-boyfriend at a science conference
Sheldon lectures at the university but worries that the students don't seem to like him. Raj fantasises about his best friend's girlfriend.
A wealthy benefactor at the university makes Leonard ponder just how far he's willing to go in the name of science.
Howard is torn between taking things to the next level with Bernadette, and pleasing his mother. Leonard rekindles his relationship with Raj's sister, Priya.
Sheldon gets a new group of friends when the gang start hanging out at Raj's more often and he realises that it's actually Leonard who is the centre of their social group and not him.
Leonard must choose between his new girlfriend Priya, and his friendship with Penny. Meanwhile, Howard drives Sheldon crazy with a magic trick.
When someone hacks Sheldon's online gaming account, the guys go on a quest to find the culprit.
Sheldon and Amy experiment on their friends by spreading gossip, while Howard takes a big step in his relationship with Bernadette.
Priya uses her legal nous to dissect the roommate agreement and put Sheldon in his place. Meanwhile, the girls take Sheldon dancing.
Raj attempts to cure his social anxiety disorder around talking to women, while Penny uses Bernadette to spy on Leonard and his girlfriend.
Howard finally tells him mum that he's engaged to Bernadette. Unfortunately, the news lands his mum in hospital.
In the series four finale, Raj moves in with Sheldon after overhearing Leonard and Priya's Star Trek bedroom fantasy