TOS Season 3 Episode 14 Whom Gods Destroy

Plot

The Federation starship Enterprise arrives at the planet Elba II, home to a Federation facility for the criminally insane, carrying a shipment of new medication which is believed will cure all violent insanity. Captain Kirk and First Officer Spock beam down with the shipment to meet the facility director Donald Cory. Instead, they find that the inmates have taken over the facility and locked Cory in a cell. The inmates are led by Garth of Izar, a former starship fleet captain, mentally unstable as the result of an injury. He acquired the ability to shapeshift using a method taught to him to heal his own injuries, and was released from his cell when he assumed Cory's form.
Kirk and Spock are imprisoned, and Garth, in Kirk's form, orders Chief Engineer Scott to beam him aboard, intending to use the Enterprise to conquer the galaxy. However, Scott refuses when Garth fails to give the correct response to a passphrase challenge. Scott and the Enterprise crew recognize something is wrong, but a force field around the facility prevents them from taking any action.
After a banquet with Kirk and Spock, Garth inquires about the passphrase. Kirk refuses to reveal it, and Garth resorts to torturing both Doctor Cory and Kirk. One of the inmates, an Orion female named Marta, attempts to seduce and then kill Kirk, but Spock arrives and subdues her. The two make contact with the ship. Kirk, sensing something amiss, instructs Spock to provide the passphrase, which he cannot do, forcing Garth to reveal he had taken Spock's form.
Once again a captive, Kirk witnesses Garth's "coronation" as "Master of the Universe". Garth demonstrates his power by killing Marta with an explosive planted on her body. Still unable to get Kirk's cooperation, Garth sends for Spock, who overpowers his escorts. On entering a control room, Spock faces two Kirks, each of whom accuses the other of being Garth. A struggle begins, during which one Kirk orders Spock to stun them both; Spock stuns the other, who is revealed to be Garth of Izar. Once again in control of the facility, Doctor Cory administers the new drugs to Garth and the other inmates. Once cured of his mental illness, Garth no longer remembers anything that has happened since his injury.