Executive Producer: Gene Roddenberry
Producers: Gene L. Coon + John Meredyth Lucas
Associate Producer: Robert H. Justman
Script Consultant: D. C. Fontana

Writer: Jerome Bixby
Dir.: Marc Daniels

Captain Kirk: William Shatner
Science Officer, First Officer, Cmdr. Spock: Leonard Nimoy
Dr. Leonard "Bones" Mc Coy: DeForest Kelly
Lt. Sulu: George Takei
Lieutenant Commander Montgomery Scott: James Doohan
Lieutenant Uhura: Nichelle Nichols
Nurse Christine Chapel: Majel Barrett
Tharn: Vic Perrin
Lt. Marlena Moreau: Barbara Luna
Farrell: Pete Kellett
Wilson: Garland Thompson
Lt. Kyle: John Winston

Stardate: Unknown 

Spock locks up. After diplomatic talks with the peaceful Halkan council on planet Halkan Captain Kirk, Dr. McCoy, Mr. Scott and Lt. Uhura have been transmitted back to the Enterprise. They arrive on board of th ISS Enterprise, in a parallel universe, because of a malfunctioning of the transmitter after atmospheric turbulence. The Parallel universe portraits the same people under the opposite conditions. 

There they see different, warlike emblems on the walls, different uniforms and definitely different attitudes: barbaric and violent. Instinctively they behave in the same way to survive the hostile environment. 

Meanwhile, on the USS Enterprise, Spock is confronted by a very upset mirrorr-captain, who can't understand what's happened. 

On the ISS Enterprise, Kirk, McCoy, Scotty and Mrs. Uhura encounter their crewmates, who greet them in a different way and seem to be used to achieve a better rank by killing their superiors; the four try to meet as often as possible on their own (without raising too much suspicion) to find a way out. Even Spock appears to be comfortable with the savage methods of this universe. He doesn't mind to apply torture on crewmembers and, when Kirk wonders about it, states that that is the best to mantain order on board. He also states his loyalty to his captain, although he explains it by telling that he feels safer as a secondary target. Kirk feels that he's not so different to his friend, as it would seem. Spock is impressed by this Kirk's unusually merciful attitute and get suspicious. 

Kirk meets Lt. Marlena, the captain's woman. Marlena, mistaking him for the Kirk she knows, welcomes him in a very friendly way and, in her chattering, reveals to him an alien tool, the Tantalus Field, that the mirror-Kirk uses to spy on his enemies and get rid of them. She locks it on Spock to discover that he has been ordered to kill Kirk and take his place, because of the captain's reluctance to destroy the Halkans. When Marlena wants to trigger the field, Kirk stops her in time. Marlena wonders at Kirk's benevolence and get suspicious. 

Kirk and his comrades arrange an escape plan, which needs to be put on action soon, before the gate between the two universes close for ever. It is a risky plan, which partially depends upon the "regular" Spock to understand what has happened and perform a similar procedure on his side. It will also require somebody to stay behind to operate the transporter. Scott volunteers, but Kirk refuse and decides he will be the sacrificed one. They gather in sickbay, but they are interrupted by an evil Sulu, who wants to kill the captain and the ones loyal to him, in order to get more power. Marlena, who's monitoring them through the Tantalus Field, eliminates Sulu, but demands to be taken with Kirk. 

Spock also finds out about the escape plans and wants to shatter them. He force Kirk ant the others back in sickbay and a struggle burst out. The four fight for their life and they manage to knock Spock out. They rushe to the transporter room, but McCoy can't be persuaded to abandon the unconsciuos Spock to his fate. He stays back and treats him, until he comes around and, by forcing a mindmeld on the doctor, finally finds out who they really are and what is actually going on. 

He is convinced by Kirk that the exchange will bring his four crew members back and will be better for all of them. Spock admits with Kirk that his captain never spoke to him as he did, Kirk shares his appreciation about both Spocks in both universes. He also convinces him that the tactics of this universe will lead inevitably to it's own destruction. Spock understands the logic in it and seems to agree: The only one who can begin the changes will be him. To overcome Spock's last hesitancies, Kirk tells him to take a look to his captain's quarters. Spock will operate the transporter to allow Kirk to go back to his univers along with the others. Marlena also understands and stays. They use an energy field and the right moment to beam back. 

Back on the USS Enterprise Spock had the other four ready for exchange. In the very moment in which Kirk, McMcoy, Scott and Uhura materialize on the right ship, they counterparts are sent back on theirs. They will find a changed situation waiting for them. Kirk wonders how he had managed to find out that quickly since most of the others had not. Spock explains that a dignified person can play the barbaric, but it won't be possible to pretend otherwise.

Pictures courtesy of www.trek5.com