The USS Enterprise is summoned to Planet Q by Dr. Thomas Leighton, Captain Kirk's friend, to explore a potential new synthetic food source. In truth, Leighton suspects that Anton Karidian, leader of a Shakespearean acting troupe performing Macbeth on the planet, is Kodos the Executioner, the notorious former governor of Earth's Tarsus IV colony. Kodos had ordered the execution of half the colony's 8,000 inhabitants amid a food shortage, with Leighton and Kirk as eyewitnesses. Kirk initially dismisses the idea but reconsiders after investigating Karidian's background.

Hoping to meet Karidian at a party at Leighton's home, Kirk encounters Karidian's daughter, Lenore, and later finds Leighton dead. Kirk arranges for the Enterprise to transport the acting troupe to their next destination and transfers Lt. Kevin Riley, another Tarsus IV massacre witness, to Engineering. These actions pique First Officer Spock's curiosity, leading him to uncover the history of the massacre, Kirk and Riley's connection, and the mysterious deaths of seven of the nine witnesses when Karidian's troupe was nearby.

After surviving a poisoning attempt and overhearing Dr. McCoy's log entry, Riley seeks vengeance against Karidian, believing him to be Kodos. Kirk stops Riley from taking action and confronts Karidian, who defends Kodos's actions without admitting to being him. A computer analysis of Karidian's voice nearly matches Kodos, but Kirk hesitates to accuse him. Eventually, Lenore reveals that she has been killing witnesses to protect her father, at which point Kirk attempts to arrest them both. Lenore tries to fire a phaser at Kirk, but Karidian sacrifices himself, and Lenore breaks down, unable to accept her father's death. Later, McCoy reports that Lenore believes her father is still alive and performing for adoring audiences.