An ion storm during a transporter beam-out exchanged Kirk, McCoy, Scott and Uhura with their counterparts from a brutal parallel universe ruled by the Terran Empire.
In 2267 Kirk, McCoy, Scott and Uhura were beaming back from failed talks with the Halkans when an ion storm disrupted the transporter. They arrived on the ISS Enterprise in a parallel universe where Starfleet served the violent Terran Empire, officers rose through assassination, and the Halkans faced destruction for refusing to hand over their dilithium. At the same time, their mirror counterparts appeared aboard the Federation Enterprise, where Spock quickly saw through them and had them held.
The landing party pretended to be their counterparts while Scott worked out how to recreate the conditions of the accident. Kirk faced an assassination plot from Chekov and a challenge from the ambitious Sulu, and he was helped by Marlena Moreau, the mirror Kirk's companion, who controlled a device called the Tantalus field. The bearded mirror Spock discovered the truth but helped them return.
Before leaving, Kirk urged the mirror Spock to think about the illogic of the Empire and the value of reform. Later events shown in Star Trek: Deep Space Nine suggested that Spock did change the Empire, though its weakening made it open to conquest.