On the recently-discovered planet Phylos, the U.S.S. Enterprise crew meets Dr. Keniclius, a human scientist who survived Earth's Eugenics Wars and now grown to giant proportions. The Phylosians are a once vast plant-based civilization that has been reduced to a few survivors. Led by Keniclius, the Phylosians abduct Spock so the doctor can make him into a giant clone with the intention of forcing peace on the galaxy.