"The Conscience of the King" is an episode of the science fiction television series Star Trek. It is episode number 13, production number 13, and aired on December 8, 1966. It was written by Barry Trivers and directed by Gerd Oswald.

The episode takes its title from the concluding lines of Act II of Hamlet: "The play's the thing/Wherein I'll catch the conscience of the king."

In this episode, Captain Kirk crosses paths with an actor suspected of having been a mass-murdering dictator many years before.

The episode featured the final appearance (in production order) in the series of Grace Lee Whitney (Yeoman Janice Rand). Whitney had already been notified that she was fired from the series a week before filming on this episode began. Her brief walk-on scene was her last scene in Star Trek before her return in Star Trek: The Motion Picture.

On stardate 2817.6, the Federation starship USS Enterprise has been called to Planet Q by Dr. Thomas Leighton, a research scientist and friend of Captain Kirk. Dr. Leighton had claimed to have discovered a new synthetic food source. As a result, the Enterprise diverts from its scheduled course to come to the planet.

Upon arriving, however, Kirk discovers that the synthetic food discovery claim was just a ruse employed by Dr. Leighton to bring Kirk to Planet Q. Leighton reveals to Kirk his true motivation is to have Kirk confirm Leighton's suspicions that Anton Karidian (Arnold Moss), the leader of a Shakespearean acting troupe currently on the planet, is in fact Kodos "the Executioner," the former governor of the Earth colony of Tarsus IV, who was responsible for the massacre of over 4,000 people—including members of both Kirk's and Leighton's families—20 years earlier.

At first, Kirk is unwilling to believe Dr. Leighton's accusations. He states he is satisfied with the official version of history that Kodos died in the aftermath of a battle between his loyalists and relief forces from Earth and that a burned body discovered in the wreckage was that of Kodos. He begins to doubt those convictions, however, when Dr. Leighton is found dead the next day under mysterious circumstances.

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