CBS

Season 1: Episode 7 - The Dumpling Paradox

The Dumpling Paradox: Penny's promiscuous friend comes to town and Wolowitz hooks up with her. This leaves the guys without their fourth wheel. Sheldon goes a little crazy because his world has been thrown off, and Penny has to stay with the guys in their apartment. With Wolowitz preoccupied, the guys discover they are without a fourth member of their group, and in desperation, they ask Penny to join their Halo team.