The Big Bang Theory Season 1 Episode 15
The Porkchop Indeterminacy
The guys meet Sheldon's extremely attractive twin sister Missy (Courtney Henggeler) who is in town for a wedding. The boys fall instantly in love with her and Leonard insists she stay with them. The gang instantly hits on her, forcing Penny to come rescue her.
Meanwhile, Leonard tries to convince Sheldon to be protective of Missy so the other guys won't have a chance. However, Leonard's plan works too well and Sheldon forbids Leonard from going out with her. When Missy learns of Sheldon's plans, she beats him up and he agrees she can date whoever she wants. The gang has a Wii Boxing match for first dibs and Leonard wins. However, Missy ends up turning him down. Wolowitz goes next and is also rejected.
Meanwhile, Raj, who has been taking part in a drug trial for medication that can cure his inability to talk to attractive women. It works but it wears off just as he's going to ask out Missy for his turn. The worst part is Missy actually liked him.