The U.S.S. Enterprise is in pursuit of an unknown alien ship which has destroyed a Starfleet base on Cestus III. In an uncharted area of space, both the alien ship and the Enterprise are caught by an advanced race called Metrons. The Metrons are angry at the two ships for trespassing into their space and believe that physical combat is the answer to finding justice. They transport Kirk and the alien captain, a lizard-like creature called a Gorn, to an uninhabited asteroid to fight to the death. The Metrons promise that the victor and his ship will be set free, while the loser will be destroyed, along with his ship and crew.

DeForest Kelley (Dr. Leonard H. McCoy)
James Doohan (Lt. Cmdr. Montgomery Scott)
Nichelle Nichols (Lt. Uhura)
George Takei (Lt. Hikaru Sulu)
Jerry Ayres (Lt. O'Herlihy)
Grant Woods (Lt. Kelowitz)
Tom Troupe (Lt. Harold)
James Farley (Lt. Cmdr. Lang)
Carole Shelyne (Metron)
Sean Kenney (Lt. DePaul)

• Lt. Harold was the only survivor of the Cestus III massacre. After the landing party beamed down, O'Herlihy and Lang were also killed.
  
• Stunt actors Bobby Clark and Gary Combs portrayed the Gorn captain in the suit, while Ted Cassidy provided the voice.
  
• This episode is the first time photon torpedoes are used. It's also the first time the "Federation" is mentioned.