The Enterprise's space scope detects a fertile green planet. Scotty comments that it looks "Kelly Green" and Captain Kirk orders a landing party to be assembled. While approaching the planet, the Enterprise passes through a mist, within which float plant spores which fasten themselves to the hull and quickly penetrate the ship. The spores cause the laboratory guinea pigs to transform into tree-like plants which attack Spock and Dr. McCoy, but are soon destroyed by security guards.