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Penny starts dating Zack again. Used to being the target of bullying, Leonard, Sheldon, Howard and Raj instead bully Zack through their implications to him that he's stupid. Not fully realizing what they're doing until Penny points it out to them, the guys decide to apologize to Zack in the only way they can. Accepting their apology, Zack decides to start hanging out with them, much to Penny's chagrin. This hanging out leads to the idea that Zack should replace Leonard as Superman in their group Justice League of America costume for the New Year's Eve party at the comic book store, with Penny the sixth wheel as Wonder Woman. These actions lead to questions from Leonard to Penny about why she's back together with Zack.