The Justice League Recombination

The Justice League Recombination is the eleventh 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 at Stuart's New Year's Eve costume party, which would replace Leonard  as that role. Leonard agrees as long as he goes as Green Lantern, leaving Raj  to be Aquaman , much to his dismay. They return home with the Justice League costumes, with Howard being Batman, Raj  as Aquaman , Sheldon  as The Flash , and Leonard  as Green Lantern. They even purchase a Wonder Woman costume for Penny. However, Penny doesn't want to go and suggests that they ask Bernadette  or Amy, but Howard  says Bernadette  is under quarantine and Sheldon  says Amy  doesn't believe in wearing costumes, leaving no choice but for Penny  to go. On the night of the party, the guys arrive at the apartment in their costumes. But 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, Sheldon  says that they'll have to go in their Muppet baby costumes but then they make Leonard  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 it becomes 2011, 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, are brave enough to stop the car theft. That New Year's Day evening, Leonard warns Sheldon  that he wants to change the thermostat temperature. Sheldon then imagines himself as The Flash, speeding to the Grand Canyon to scream in frustration. Once he is finished, Sheldon tells Leonard, "Fine".

Trivia
The Justice League consists of Sheldon asks in what universe is wonder woman blonde, in fact Cathy Lee Crosby starred as a blonde Wonder Woman in the unsuccessful 1974 TV movie "Wonder Woman "
 * Sheldon as: The Flash
 * Leonard as: Green Lantern
 * Howard as: Batman
 * Penny as: Wonder Woman
 * Zack as: Superman
 * Raj as: Aquaman