Tharn
Vic Perrin

Lt. Marlena Moreau
Barbara Luna

Farrell
Pete Kellett

Wilson
Garth Pillsbury

Lt. Kyle
John Winston

Following an unsuccessful mission to persuade the Halkans to supply the Federation with dilithium crystals Kirk, Uhura, Scott and McCoy beam back to the Enterprise during an ion storm. On re-materialising they appear to be wearing different uniforms and are met by a Mr. Spock who is sporting a beard. On being asked the status of the mission, Kirk carefully replies 'no change', at which Spock orders Sulu to prepare the ship for a phaser barrage of the planet. Kirk makes an excuse to get the landing party to sickbay for a checkout and, on the way, they notice subtle changes to the ship and the fact that every member of the crew is armed.

By themselves, Kirk voices the possibility to the other three that they have been, somehow, switched into an alternative universe by the effects of the ion storm on the transporter. He orders Scotty to sabotage the ship's phasers to protect the Halkans and Uhura to the bridge to find out what his standing orders are. He and McCoy access the sickbay computer terminal to try and get more information about their situation. On the bridge, a heavily scarred Sulu harasses Uhura who is saved by Kirk's arrival. She manages to convey to him that his orders are to destroy the Halkans unless they agree to part with their dilithium.

The Enterprise passes the first firing point without Kirk giving the order to fire and Spock orders Sulu to lock on to the secondary target, only to be countermanded by Kirk who orders the phaser banks onto standby. He tells Uhura to get McCoy and Scott to meet them in his quarters and leaves the bridge, followed by Chekov. On leaving the turbolift he is ambushed, but one of the guards decides to swap sides and stuns Chekov before he can kill Kirk. Meanwhile, in the 'other' universe, the mirror versions of the landing party have been behaving so out of character that Spock has guessed what has occurred and has imprisoned them.

Scotty tells Kirk that he can reproduce the transporter conditions that got them there but the changes he needs to make will show up on Sulu's security system, therefore some sort of diversion will be necessary. Kirk delegates the diversion creation to Uhura and returns to his quarters, finding a beautiful girl called Marlena on his bed when he arrives. Apparently she is the 'captain's woman' and tries to find out what his plans are. Spock calls him and tells him confidentially that he has received orders to kill Kirk and take his place if the Halkans are not subdued by dawn.

Marlena tries to persuade him to use the Tantalus device on Spock. This, apparently, is a device which can monitor and kill from a distance and is the secret of the captain's power. Kirk refuses and she leaves, following which he contacts Scotty and explains the situation, telling him that they need to be able to beam home within half an hour. Spock suspects what has happened and has been watching the engineering activities carefully while also noticing that he, himself, is under scrutiny from Sulu.

Uhura manages to distract Sulu from his security board while Scotty makes the necessary changes but Spock sees what is happening and goes to intercept Kirk in the transporter room. Spock takes him back to sickbay where the others have congregated but they manage to overpower him, injuring him in the process. McCoy insists on treating him before they can leave. Due to the delay, Sulu has had time to discover their whereabouts and confronts them, telling them that he plans to set them up to look as if they have killed each other, leaving him in charge of the Enterprise.

In Kirk's quarters Marlena has been monitoring events using the Tantalus field and uses it to kill Sulu's henchmen one by one. Kirk overpowers the astonished Sulu and disables him, then joins the others in the transporter room while McCoy finishes treating Spock's injuries. Spock manages to get close enough to McCoy to initiate a mind-meld, confirming his guess about the situation. He goes with McCoy to the transporter room and agrees to operate it for them as he wants his own captain back.

Before leaving, Kirk tells Spock that their empire must eventually fall and that it would be illogical for him to be a part of it. Marlena, who cannot be taken back with them, despite her pleas, due to the power balance of the transporter, agrees to show Spock the Tantalus device which will allow him to gain control of the Enterprise and initiate change. Spock says he will consider Kirk's suggestion.