The Big Bang Theory Episode 1.05 The Hamburger Postulate

While eating at the restaurant where Penny works, Leonard and Sheldon run into Leslie, a fellow scientist who works with Leonard. Leslie suggests Leonard play in her string quartet, but what she really wants is to get up close and personal on another level. When Sheldon comes home and finds a necktie on Leonard's door, he drops in to Penny's apartment to her to ask what the necktie means. When Leonard finds out Sheldon has told Penny about his rendezvous with Leslie he is furious. Fearing an imagined love triangle between the two women and himself, Leonard is disappointed to realize all Leslie was interested in was a one-time event.