Lt. Leslie
Eddie Paskey

Leila Kalomi
Jill Ireland

Elias Sandoval
Frank Overton

Lt. Commander Kelowitz
Grant Woods

Painter
Dick Scotter

Lt. Vincent DeSalle
Michael Barrier

The Enterprise arrives at Omicron Ceti III to investigate a lack of contact with the colony there and Kirk leads a landing party down to the planet's surface. They expect to find no survivors due to their having been exposed to lethal Berthold rays and are amazed to be met by the leader of the colonists, Elias Sandoval, who tells them that their sub-space radio has been out of action.

Sandoval introduces them to Leila Kalomi, who knows Spock from six years previously, and takes them around the colony where they notice the simple life of the people and the strange lack of animals. On examination, McCoy records that all the colonists are in a condition of perfect health even to the point that Sandoval has re-grown an appendix. Kirk receives orders from Starfleet to evacuate the colony but Sandoval refuses to leave and Leila promises to show Spock the reason for their exceptional happiness and health.

She leads him to some strange alien plants that throw out their spores, covering the Vulcan and causing him to scream in pain. When he recovers, he finds that he has lost control over his emotions and kisses Leila. Kirk tries to contact him but he is otherwise engaged, lying with his head in Leila's lap and looking at the shapes in the clouds. When he eventually answers, Kirk is puzzled by Spock's off-handed responses to his requests for information and also his refusal to reveal his location. McCoy is left in charge of the evacuation plans while Kirk and Sulu search for Spock.

They find him swinging from a tree branch saying that there will be no evacuation. Annoyed, Kirk orders him to be arrested. As they travel back to the settlement Spock leads them to the alien plants which cover them all in spores. Kirk seems to be unaffected but the other crewmen tell him that they are mutinying and staying on the planet. On returning to the colony Kirk finds McCoy transporting some of the strange plants up to the ship. He beams up himself and finds that the entire crew is now under the effect of the spores.

Eventually they all beam down to the planet, leaving Kirk alone aboard the Enterprise. The spores start to work on him and he begins packing, but the sight of his souvenirs and the hold that the Enterprise has over him is such that the thought of having to leave makes him angry and annoyed. He recovers his senses and realises that the negative emotions have counteracted the effects of the spores. He contacts Spock and asks him to come up and help him pack some things that will be useful on the planet.

When Spock arrives, Kirk starts insulting him and provokes him into a fight. The Vulcan gains the upper hand and is about to give Kirk a severe beating when his aroused negative emotions also free him from the effects of the spores. They arrange a sonic bombardment of the planet using a subsonic frequency to make everybody irritable which soon has the desired effect. When Sandoval recovers, he realises that the colony has become stagnant and unproductive. The colonists agree to pack up and evacuate to the Enterprise.