The Big Bang Theory Season 4 Episode 11
The Justice League Recombination
Penny begins dating Zack again, and the boys make fun of his stupidity to the point of hurting his feelings. When they apologize to him, they take him with them to the comic book store where he learns about the costume-themed New Years Eve party there. The guys need him to be their Superman and Penny to be their wonder woman to win. It's Leonard that convinces Penny to go and when midnight strikes Penny kisses Zack, but you can tell she wishes she was kissing Leonard. Oh and don't worry, they won the contest.