The Big Bang Theory Season 3 Episode 21
The Plimpton Stimulation
Sheldon invites a famous female physicist, Dr. Plimpton (Judy Greer) to stay at their place for the weekend while she's in town applying for a job. She ends up sneaking behind a sexually uninterested Sheldon's back and sleeping with Leonard. Penny finds out the next morning that Leonard slept with her and is clearly bothered by her ex moving on so fast. Soon Plimpton begins hitting on anything that moves and tries to have an orgy with Leonard, Wolowitz and Raj, but ends up with just a Raj.