A once-great Starfleet hero is a patient in an asylum where inmates have taken control.
The Enterprise delivers medicine to an asylum for the criminally insane on Elba II and finds that the inmates have taken over. Garth of Izar, a legendary Starfleet captain who was driven mad, has taken control, and he can change his shape to pretend to be Kirk. Kirk and Spock must find a way to stop him.
The episode gives a view of the possible fall of a hero, as Garth's skill makes him a dangerous opponent. It features a famous scene of a shape-shifting villain, and the story includes a memorable guest performance by Steve Ihnat. It is remembered as an eerie, theatrical hour.