The Big Bang Theory Season 1 Episode 6
The Middle-Earth Paradigm
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. He takes off and heads home and Penny comes to check on him and apologize. She kisses Leonard and wonders why more men can't be like him. Penny goes to leave and they kiss once more.
Meanwhile, Wolowitz can't find Rajesh, who went home with a woman who thought he was a good listner (Rajesh is too shy to talk around women).