The Justice League Recombination

The Justice League Recombination is the tenth episode of the fourth season of the Big Bang Theory.

Plot Summary
The guys get Zack and Penny to join their Justice League costume for a New Year's Eve bash at the comic book store.

Expanded Plot
The guys discover that Penny and her ex boyfriend, Zack, are back together and they take advantage of his lack of intelligence and make fun of him. After Zack finally realizes what's going on and storms out, the guys decide to apologize to him. Upon receiving Milk Duds, Zack immediately forgives Leonard and his friends and goes with the guys to the comic book store, leaving Penny behind. At the store, the guys realize that Zack would be great as Superman for their Justice League group costume. They return home with the Justice League costumes, with Howard being Batman, Raj Aquaman , Sheldon The Flash , and Leonard as Green Lantern. They even purchase a Wonder Woman costume for Penny. However, Penny refuses to wear the black wig and eventually refuses to even go to the New Year's Eve costume party at the comic book store. None of the guys, not even Zack, can convince Penny to go. Since Zack would not go without Penny and the Justice League wouldn't be complete without Superman and Wonder Woman, Leonard is forced to talk to Penny. Penny reveals that she is only with Zack because she doesn't want to be alone for New Year's Eve. It is hinted that she still hasn't fully gotten over Leonard. Penny relents, puts the wig on, and the group later wins the costume contest. When midnight strikes and Zack is kissing Penny, she looks guiltily over at Leonard for the duration of the entire kiss. The episode concludes with the group witnessing an ongoing car theft on the way back home and despite wearing Justice League costumes, they comically scurry off, none of them, not even Zack, brave enough to stop the car theft.

Trivia
The title refers to Sheldon, Leonard, Howard and Raj inviting Penny and Zack to be Wonder Woman and Superman in their Justice League team.