Ah, the old switcheroo. Mirror, Mirror is a TOS episode that is awesome as it is silly. Another bit of proof that transporters can do fucking anything, this particular piece of television revealed to humanity one one of the most important lessons of the the 20th century - that anyone with a goatee is utterly evil. If you ever see someone with a goatee, stone them to death. You'll be saving lives that way.
Crap That Happened Edit
The Halkans, some nice people living on a planet made of dilithium, are being begged by Captain Kirk to let the Federation mine thier planet. The Halakns refuse and blah blah blah suddenly the away team is in an evil, mirror universe.
Uhura, Kirk, McCoy and Scotty blah blah blah seeing the horror of the Terran Empire blah blah blah Empress Hoshi blah blah blah Tantalus Field blah blah blah Mirror Spock, the Space Pirate, has orders to kill Kirk blah blah blah blah blah blah Kirk and Co. get back home safe.
Sorry. My toddler's attention span kicked in.