While exploring an uncharted region of space, the U.S.S. Enterprise comes upon an alien space buoy, warning ships away and blocking the starship's path. Kirk's orders the phasers to destroy the buoy but immediately an alien ship, the I.S.S. Fesarius, shaped like a large, glowing crystal traps the starship. The Fesarius captain Balok tells Kirk that the Enterprise has trespassed and committed hostile actions, and therefore, must be destroyed. Employing a trick from the Earth game of poker, Kirk tries a desperate bluff by convincing Balok that if the Enterprise is fired upon, the "corbomite" in the starship's hull will self-destruct and destroy both ships.

Anthony Call (Lt. Dave Bailey)
Clint Howard (Balok)
DeForest Kelley (Dr. Leonard H. McCoy)
Grace Lee Whitney (Yeoman Janice Rand)
George Takei (Lt. Hikaru Sulu)
James Doohan (Lt. Cmdr. Montgomery Scott)
Nichelle Nichols (Lt. Uhura)

• McCoy gives Kirk a physical and then puts him on a salad diet.
  
• This episode moves along at a very slow pace; The Enterprise does not destroy the bouy until nearly halfway through the episode.
  
• Bailey remains behind with Balok as part of an exchange program so that Balok can learn more about the Federation, at Kirk's suggestion for a volunteer. Bailey seems an odd choice, since the stress of the [not very stressful] encounter rendered him unable to perform his duties.
  
• Captain Kirk announces the ship as the United Earth Ship Enterprise. (It is later established that Starfleet is a division of the United Federation of Planets.)
  
• Uhura wears a gold command division uniform in this episode instead of her normal red uniform.