“Mirror, Mirror”
“Mirror, Mirror” Kirk, Uhura, Scotty and McCoy are accidentally transported into a parallel universe aboard the I.S.S. Enterprise, while their brutal counterparts of the parallel universe are beamed aboard the U.S.S. Enterprise. Reluctantly the four officers try to fit in their unusual roles. When they attempt to return to their world, they get unexpected help from the parallel Spock who is a sincere person in both universes. summary by Ex-Astris-Scientia
The very first of the whole series. I wonder why the concept became so popular over the years? I mean, each and every series has at least one episode (I believe DS9 has a few) to do with the mirror universe.
Oh, well, I admit, it’s quite interesting to peep through the looking glass (Whoops! Pun not intended) and see what you are like there.