Kirk suspects a travelling actor is a mass murderer from his past on Tarsus IV.
Kirk's old friend Dr. Thomas Leighton lures the Enterprise to Planet Q to claim that Anton Karidian, head of a touring Shakespeare company, is really Kodos the Executioner. As governor of Tarsus IV during a famine, Kodos had about half of the colony's 8,000 people put to death, and Kirk was among the few who saw his face. Leighton is soon found dead, and Kirk arranges passage for the troupe while Spock learns that seven of nine eyewitnesses have died wherever the players appeared.
Kevin Riley, another Tarsus survivor, is poisoned and a phaser is rigged to overload in Kirk's quarters. Karidian never confesses but a voice analysis points to Kodos. During a shipboard Hamlet, Kirk talks Riley out of revenge, and Karidian's daughter Lenore reveals she has been killing the witnesses. Karidian dies taking her shot meant for Kirk, and Lenore loses her grip on reality. Barry Trivers wrote this first-season story, which takes its title from Hamlet.