The U.S.S. Enterprise is scanned by a powerful energy source coming from the planet Excalbia. The starship had been sent to survey the planet, but it was thought to consist of nothing but a lavalike surface without inhabitants. An image of Abraham Lincoln appears in space and requests to be beamed aboard, where he invites the crew to the planet. Kirk, Spock and Lincoln are joined on the surface by an image of the Vulcan Surak. A rock-creature appears and introduces Kirk and Spock to four more illusionary figures from history — this time the fiercest conquerors, tyrants and villains of the past, from Genghis Khan to Kahless the Unforgettable of Klingon. The creature forces the two sides to fight for the lives of the Enterprise crew.

James Doohan (Lt. Cmdr. Montgomery Scott)
Lee Bergere (Abraham Lincoln)
Barry Atwater (Surak)
Phillip Pine (Colonel Green)
George Takei (Lt. Hikaru Sulu)
Nichelle Nichols (Lt. Uhura)
Walter Koenig (Ensign Pavel Chekov)
Arell Blanton (Lt. Dickerson)
Carol Daniels Dement (Zora)
Robert Herron (Kahless)
Nathan Jung (Ghengis Khan)

• This episode first introduces Surak of Vulcan and the Klingon leader Kahless the Unforgettable, who are both mentioned in later series and films. Both are revered figures to their respective races and led them down their respective paths.
  
• This is the last appearance for Uhura, who does not appear in the series' last two episodes.