The U.S.S. Enterprise again crosses paths with Cyrano Jones, who this time has tribbles that do not breed but instead grow to an enormous size. The tribbles are back and are causing problems for the Klingons.

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.