The Big Bang Theory Episode Guide
The guys are invited to Penny's Halloween party and decide to go only when the idea of costumes are mentioned. At the party, Wolowitz and Leonard attempt to socialize until Penny's ex, Kurt, shows up and humiliates Leonard.
While eating at the Cheesecake Factory where Penny works, the guys run into Leslie Winkle, the girl who previously rejected Leonard. Leslie asks Leonard to play in her string quartet as the cellist, but what she really wants is him.
When Leonard witnesses Penny kissing a guy in front of their apartment, he is hurt and feels "rejected." The guys try to convince him to ask a girl out at work and when he's rejected by Leslie Winkle, he is even more depressed.