While transporting during an ion storm, Kirk, McCoy, Scott, and Uhura are switched with evil counterparts in a parallel universe, and it is up to the parallel Spocks to get their respective crew members back.
Air Date: 10/06/67
Director: Marc Daniels
And here we have the reason why Season 2 is the best season of the original series. Because of this episode, everyone knows that goatees mean you are evil! This is a truly exceptional episode...hands down. The overall peril Kirk and company face is nerve-wracking, and the acting is terrific. Let's face it, everyone has a person or two they would like to put in the agony booth, right? I would easily list this one in my top ten TOS episodes.