Sulu and another crewman are investigating a primitive but ancient-Earth-like alien culture incognito, but their disguise is blown and they're running for their lives. The Enterprise can't beam Sulu up in time, and he is hit by a weapon on the planet which leaves him under the control of something or someone called Landru. Kirk and Spock beam down, finding the planet's people engaged in unusual rituals, and also finding out from some of the locals that Landru has complete control over most everyone on the planet, aside from a small resistance effort. What begins as an effort to free the people on the planet becomes a matter of survival when Kirk and Spock become hunted by Landru's puppets.
