While visiting the recently discovered planet Phylos, the U.S.S. Enterprise landing party discovers an apparently abandoned city. While exploring the city Sulu picks up a plant and is poisoned. As the landing party tries to save Sulu, the alien species that inhabit the planet approach and save Sulu's life. Upon further investigation of the alien creatures, who are plantlike beings called Phylosians, the crew discovers that the poison that affected Sulu was accidentally brought to the planet by a visitor who subsequently worked to find a cure. The Phylosians call this visitor "Master" and "Savior."

The landing party eventually discover that this visitor was actually a Terran scientist by the name of Dr. Starros Keniclius who has since died. Now a giant clone of Dr. Starros Keniclius, by the name of Keniclius Five, has taken his place. The giant wishes to clone Spock to create an intergalactic peacemaker. Ignoring Kirk's insistence that the Universe has become a virtually peaceful conglomeration of worlds, Keniclius Five clones Spock, crippling the Vulcan almost to the point of death. Kirk realizes the only way to save Spock is to recite aspects of Vulcan philosophy on life and death to Keniclius Five. Kirk convinces the clone that the new Spock is but a copy and not the original. The clone Spock saves the other's life through a Vulcan mind meld and the duplicate remains on the planet with Keniclius to revitalize that world's civilization.

Cast:

William Shatner as James T. Kirk
Leonard Nimoy as Spock
DeForest Kelley as Leonard H. McCoy
James Doohan as Montgomery Scott
Nichelle Nichols as Uhura
George Takei as Hikaru Sulu

Guest Cast:

James Doohan as Lieutenant Arex
James Doohan as Dr. Keniclius
James Doohan as Agmar
James Doohan as Morgan
James Doohan as Kolchek

Creative Staff:

Written By: Walter Koenig
Director: Hal Sutherland