On the planet Phylos, a human scientist from the era of Earth's Eugenics Wars kidnaps Spock to make a giant clone that will impose peace on the galaxy.
While the crew surveys the newly found planet Phylos, Sulu is poisoned by a walking plant, and the plant-like Phylosians cure him before kidnapping Spock. Their master is Dr. Stavos Keniclius, a human scientist from the time of Earth's Eugenics Wars who has survived for centuries by moving his mind into new clone bodies, the latest of them a giant.
Keniclius plans to enforce peace across the galaxy with a giant clone of Spock and a fleet of Phylosian ships. The clone saves the original Spock with a mind meld, and the two Spocks persuade Keniclius that his plan is not needed. Walter Koenig wrote the episode, the first Star Trek script by a cast member.