The USS Enterprise, led by Captain Kirk, reaches planet Beta III in the C-111 system to investigate the disappearance of the USS Archon nearly a century ago. After beaming up from the planet, Lt. Sulu displays unusual euphoria and refers to the crew as "Archons." Kirk and his team beam down and find the inhabitants living in a 19th-century Earth-like society governed by cloaked "Lawgivers" and a secretive dictator, Landru, with periodic episodes of chaos known as "Festival."

Kirk's party takes refuge in Reger's boarding house but is soon pursued by Lawgivers. After evading capture, they learn from Reger that Landru destroyed the Archons. Meanwhile, the Enterprise is attacked by heat beams from the planet, putting the ship at risk of crashing within 12 hours if the beams aren't disabled.

Kirk and his team are captured and imprisoned, with Dr. McCoy under Landru's mental control. They are rescued by Marplon, an underground resistance member against Landru, who reveals Landru's historical role in shaping their society. After discovering Landru, a computer programmed by its deceased creator, Kirk, and Spock convince the computer to self-destruct due to its stifling of creativity and free will.

The heat beams cease, saving the Enterprise. Kirk agrees to leave Federation advisors on Beta III to help rebuild the civilization.