To Sheldon, Leonard, Raj, and Howard, Zack was beneath them and worthy of being made fun of. In a strange twist, Zack not only agreed to go with them to the comic book store, but to also join them in their costume contest. For Penny, this strange twist of events, especially after Zack referred to the four as his friends, gave Penny a reason to retreat to her own apartment, even after she dressed up as Wonder Woman. Penny not having changed out of her costume was a subtle indication of how she was really feeling.
First Zack attempted to convince Penny to go with them to the costume contest, but to no success. Next, Sheldon, with Flash powers, made an attempt to persuade Penny. His factual correctness combined with his complete inability to empathize with Penny also yielded no positive results. Finally, Leonard decided to try to talk to Penny. There is an irony and unexpectedness, in Penny's revelation to Leonard, when she stated that she only got back with Zack, as she did not want to be alone on New Year's Eve. Leonard did convince Penny to go to the comic book store, and they ended up winning the costume contest.
Most of the great parts of this episode were the comic book references. Sheldon had some great physical comedy parts, Raj absolutely hated dressing up as Aquaman, Leonard wore his glasses over the Green Lantern mask, and Howard combined a 1960s Batman costume with the notoriously bad voice of The Dark Knight. Yet, beyond the goofiness and geek cultural references were the more important unspoken parts occurring between Penny and Leonard. The look on Leonard's face as they celebrated the New Year, and Penny's bottling up spoke more than either character could ever say to each other.