When Raj's sister has a one-day layover he forbids the others from hooking up with her, unaware she and Leonard already did long ago.

Leonard is depressed to realize all the other guys have girlfriends, while he doesn't. Amy wants to introduce Sheldon to her mother.

Wolowitz tries to get back together with Bernadette. Sheldon and Raj go to war at the university.

When Sheldon "breaks up" with Amy, the guys worry when he starts replacing her with cats.

Sheldon calculates that he'll be dead before technology exists to download his consciousness into a robot, so he goes on a binge of healthy living and ultra-safety.

Howard uses a robotic arm for his own pleasure. Penny assists Sheldon with his first date.

Leonard's idea for a smartphone app derails his friendship with Sheldon.

To secure needed funding for their school, Sheldon convinces Leonard to get friendly with a wealthy benefactor.

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Season 4 Episode 15 The Benefactor Factor Megavideo