The Enterprise travels to the planet Elba II, home of the last asylum for the criminally insane, to deliver a serum that should cure all of its remaining inmates. Kirk and Spock beam down to the planet’s surface where all seems in order, but they soon find the inmates now run the asylum, led by Garth (at one time a starship Captain, whose exploits were required reading at the Academy). Garth, who’s learned how to shape-shift, can take on the appearance of anyone, including Kirk or Spock. In the process of learning this ability, he lost his sanity. Garth plans to pose as the Captain, beam up to the Enterprise and take over the ship, but Kirk has a roadblock set up to overcome.