On a frozen world Nurse Chapel searches for her fiancé and finds androids who want to replace the crew.
Nurse Christine Chapel joins Kirk on a visit to the frozen planet Exo III, hoping to find her fiance, the noted exobiologist Dr. Roger Korby, who vanished years earlier. Korby is alive in caverns beneath the surface, where he has found machinery left by a lost civilisation that can build androids.
Korby uses it to make an android double of Kirk and plans to spread androids through the galaxy. His creations include Andrea and the towering Ruk, played by Ted Cassidy. Kirk turns the androids against their maker, and Korby himself proves to be an android copy, leaving Chapel to remain with the Enterprise.