Kirk's crossing into the mirror universe

In 2267, after failed negotiations with the Halkans for their dilithium, a transporter malfunction during an ion storm sent Kirk, McCoy, Scott and Uhura into a parallel universe. Their counterparts from that reality were sent to the prime Enterprise in their place. On the other side they found the ISS Enterprise, a warship of the Terran Empire, where officers rose through assassination and loyalty was enforced by fear.

Posing as their brutal doubles, the four had to blend in while finding a way home. The mirror Spock, bearded and as logical as his counterpart, soon saw through them but chose to help, calculating that it served his interests. Meanwhile, on the prime Enterprise, the impostors were quickly unmasked by Spock and confined.

Before returning, Kirk urged the mirror Spock to consider that the Empire was illogical and doomed, and that he could change it. Later series reveal that Spock did try to reform it, with dire consequences for the Terrans. Long seen as humanity's first visit to the mirror universe, this episode was later preceded on screen by earlier crossings shown in Enterprise and Discovery.