While exploring near the center of the galaxy, the U.S.S. Enterprise is caught inside an energy/matter vortex and all her computer systems fail. A being named Lucien appears on the bridge, repairs the ship's systems and takes the crew to explore his planet, Megas-Tu.

William Shatner (Captain James T. Kirk)
Leonard Nimoy (Commander Spock)
DeForest Kelley (Dr. Leonard H. McCoy)

George Takei (Lt. Hikaru Sulu)
Nichelle Nichols (Lieutenant Uhura)
Majel Barrett (Nurse Christine Chapel)
James Doohan (Lt. Cmdr. Montgomery Scott)
Ed Bishop (Prosecutor)

• James Doohan provided the voice for Lucien.
  
• The inhabitants of Megas-Tu, the Megans, claim to be the puritans burned at the Salem Witch Trials, and Lucien is Lucifer.