While visiting the newly discovered planet Phylos, Sulu picks up a walking
plant, called a Retlaw (Walter spelled backwards), and is poisoned. The alien species that inhabit
the planet, who are plantlike beings, approach and save Sulu's life.
From them the crew discovers that most of the Phylosians were wiped out by a plague
brought to the planet by a Terran scientist named Dr. Starros Keniclius. A
giant clone of Keniclius, named Keniclius Five, kidnaps Mr. Spock in order to clone
him and create an intergalactic
peacemaker, at the expense of the original Spocks life.
This episode was written by Walter Koenig, who played Chekov in the original series.