Evidence: Spock in this episode.
While negotiating with the peaceful Halkan people, Kirk, McCoy, Scotty and Uhura (what the hell was Uhura doing on a landing party, you might ask) beam up to the ship during a storm. The storm affects the transporter, shifting the landing party from our universe to an alternate one that is totallitarian. Kirk in this universe is more brutal and violent and the ship runs on assignation of superiors to move up in rank. In fact, Kirk faces attempted assignations twice in the course of an hour.
The Halkans are still peaceful in the mirror universe and Kirk puts himself in danger when he doesn’t just wipe them out but gives them time to consider.
Our four heroes are in a race against time to get back as the doorway between the two universes will close forever, meaning Scotty has to work yet another miracle with the transporter.
A solid, entertaining and disturbing episode. It’s interesting to see how easily Kirk can slip into the persona of evil Kirk here, at least enough to fool most of the people. Spock is not fooled and quickly figures out something is up in both universes. This is probably one of the more influential classic series episodes and led to DS9 going back to the mirror universe on a regular basis.
Now, sure there are a whole lot of coincidences that have to happen to lead the exact same set of characters to be in the exact same places in both universes. But this one is so much fun you just need to turn the brain off and go with it. It also ends with Kirk challenged the mirror Spock to lead the revolution and possibly overthrow the regime of this dark universe–a theme that is followed up on in the Shatner-verse novels as well as on DS9.
Sure it takes some work to get there, but “Mirror, Mirror” is a just plain fun to watch.