Anton Karidian / Kodos
Arnold Moss

Lenore Karidian
Barbara Anderson

Lt. Leslie
Eddie Paskey

Lt. Kevin Riley
Bruce Hyde

Lt. Matson
David Troi

Martha Leighton
Natalie Norwick

Dr. Thomas Leighton
William Sargent

During a visit to Planet Q Dr. Thomas Leighton confides to Kirk that he believes the leader of a visiting theatre company, Anton Karidian, is actually a man known as Kodos the Executioner. Twenty years previously, on planet Tarsus IV, Leighton and Kirk had survived a massacre ordered by Kodos who, since then, had been believed to be dead.

After much discussion Kirk starts to believe that Leighton may be right. Later, while walking with Karidian's attractive daughter Lenore, Kirk comes across Leighton's murdered body, which adds weight to his suspicions. In order to investigate more thoroughly he calls in an old debt from the captain of a transport ship and arranges for the vessel to miss its call at Planet Q, leaving the company stranded. Kirk offers the services of the Enterprise to transport them to their next destination.

On searching the library computer Kirk discovers that another member of the Enterprise crew, Lieutenant Kevin Riley, is also a survivor of the massacre and, for his protection, orders Spock to transfer him to Engineering. Without any logical explanation for the order, Spock starts his own enquiries and soon finds the connecting link. As he digs deeper into the computer records he discovers that nearly all of the people who could identify Kodos are now dead and that all their deaths occurred while the Karidian Company were nearby.

Things start to heat up when Riley is hurried to sickbay after being poisoned and Kirk narrowly avoids death from a hidden phaser set to overload. The evidence seems strong enough for Spock but Kirk insists that there must be undeniable proof. As payment for their ride, the actors put on a performance of 'Hamlet' for the crew of the Enterprise. Riley arrives from sickbay intent on killing Karidian but is intercepted by Kirk. On stage, Karidian overhears their confrontation and is strangely distraught by the claims.

Karidian's daughter, Lenore, reveals that she is responsible for all the murders, which she carried out in order to protect her father, and seizes a phaser to protect herself. In her haste and anxiety she accidentally activates the weapon and kills her father. This act finally breaks the delicate hold she has on sanity and, with Kodos dead and Lenore sent to a prison for the criminally insane, Justice is finally done.