The Enterprise is visiting a Federation criminal facility on the planet Elba II. Kirk and Spock beam down to provide medical supplies to the facility director, Donald Cory. Unfortunately, they find the inmates have taken over the facility. The inmates are led by a former Starship Captain, Garth of Izar. Garth is mentally unstable and dangerous because he has acquired the ability to shapeshift.

Garth takes Kirk and Spock prisoner. In Kirk's form, he orders Scotty to beam him aboard, intending to steal the Enterprise. However, Scotty refuses when Garth fails to respond correctly to a security challenge. Scotty recognizes something is wrong but cannot do anything, as a force field surrounds the entire facility.

Garth eats a meal with Kirk and Spock and inquires about the security passphrase. Kirk and Spock refuse to reveal it. In an attempt to get their cooperation, Garth tortures Doctor Cory and Kirk. An Orion female named Marta attempts to seduce and kill Kirk but is saved by Spock's arrival. Kirk learns that the Spock who just saved him is Garth in disguise.

Garth kills Marta with an explosive planted on her body, demonstrating his cruelty. Kirk still refuses to cooperate. Upon Spock's arrival, he is confronted by two Kirks. Both Kirks claim to be the real Kirk, and Spock must decide who is who. After a struggle, one of the Kirks orders Spock to stun them. Upon hearing this order, Spock stuns the other Kirk, who is actually Garth of Izar. Doctor Cory administers the new medication to Garth and the other inmates, who are cured of their mental insanities.