Caught in the beginnings of an ion storm, Kirk, McCoy and Uhura interrupt their negotiations with the Halkans for dilithium crystals, to return to the U.S.S. Enterprise. Scotty beams the landing party aboard as a burst from the storm hits the starship. The transporter malfunctions, sending Kirk, McCoy, Scotty and Uhura into an alternate universe, where they soon discover the "Galactic Empire" is maintained by fear and assassination. Now, aboard the Imperial Starship Enterprise, the four must find a way to remain undetected until they can return to their own universe. Meanwhile, the parallel versions of Kirk, Scott, McCoy and Uhura have been beamed on board the positive U.S.S. Enterprise.

Barbara Luna (Lt. Marlena Moreau)
James Doohan (Lt. Cmdr. Montgomery Scott)
George Takei (Lt. Hikaru Sulu)
Nichelle Nichols (Lt. Uhura)
Vic Perrin (Tharn)
Walter Koenig (Ensign Pavel Chekov)
John Winston (Lt. Kyle)
Garth Pillsbury (Crewman Wilson)
Pete Kellett (Crewman Farrell)

• The Mirror Universe is seen again in the Deep Space Nine episode "Crossover".