CBS

Season 3: Episode 21 - The Plimpton Stimulation

The Plimpton Stimulation: When Sheldon meets a famous female physicist online, and invites her to come visit and stay at his apartment, Leonard quickly realizes that it is the famous and very attractive female physicist that he has a big thing for. It turns out that she is interested in Leonard, and pays him a visit to his room in the middle of the night. The next day penny gets wind of this, and criticizes Leonard for jumping into bed with someone else so quickly. And Sheldon feels betrayed because Leonard is "playing with HIS friend."  It turned out that this woman played by Judy Greer is a bit of a hussy, and comes on to all the guys, culminating in a hilarious scene where she proposes a foursome. The boys back out.