The landing party finds a world governed by a computer, Landru, and a placid society that erupts each night.
The Enterprise follows the missing starship Archon to a planet where the people live peacefully by day but erupt into riots during a nightly event known as the Festival. Kirk and his landing party learn that the society is ruled by a being called Landru.
The story pits the crew against a machine that has controlled a society for thousands of years. Kirk argues with the computer until it destroys itself, a method he used again in later episodes. It is an early example of the Prime Directive being tested, and fans remember the eerie, robed citizens and their greeting.