The U.S.S. Enterprise arrives in an uncharted area of space to answer a distress call from the U.S.S. Defiant. The starship is visible on their viewscreen, but sensors say it's not there. Kirk, Spock, McCoy and Chekov beam aboard and spread out to investigate and find everyone aboard is dead, apparently killed in a bizarre mutiny. A power loss partially disables the Enterprise transporter, but the landing party manages to beam back to the Enterprise — except Kirk, who suddenly disappears along with the Defiant. As the Enterprise begins to experience the same problems that doomed the Defiant, an alien vessel appears and demands that they leave Tholian territory or be destroyed.

James Doohan (Lt. Cmdr. Montgomery Scott)
George Takei (Lt. Hikaru Sulu)
Walter Koenig (Ensign Pavel Chekov)
Nichelle Nichols (Lt. Uhura)
Majel Barrett (Nurse Christine Chapel)
Sean Morgan (Lt. O'Neil)
Barbara Babcock (Tholian voice/Cmdr. Loskene's voice)

• The alternate universe Tholians will capture the U.S.S. Defiant NCC-1764 in the Star Trek: Enterprise episode "In a Mirror, Darkly".