At the same time McCoy discovers that he has a year to live, the U.S.S. Enterprise encounters an asteroid which is determined to be artificially propelled. Kirk leads Spock and McCoy on a landing party mission to the asteroid-like vessel called Yonada and find an idyllic world home to the people ruled by Natira, a priestess who takes her orders from the central computer, "Oracle." While Kirk and Spock search for the central controls that will redirect the ship, McCoy and Natira fall in love.

Katherine Woodville (Natira)
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)
Byron Morrow (Admiral Westervliet)
Jon Lormer (Old Man)

• The Enterprise saved a planet from an asteroid 5 episodes ago in "The Paradise Syndrome".