When the Enterprise speeds to the agricultural colony Omicron Ceti 3, they expect to find none of the 150 men, women, and children colonists alive, as the planet is being bombarded with deadly and newly discovered Berthold rays. Kirk, Spock, McCoy, DeSalle, and another crewman are surprised indeed to find the colonists and their leader Elias Sandoval alive and in perfect health. Too perfect health, Bones discovers, when he finds that Sandoval now has a healthy appendix despite the fact that records show it to have previously been removed in an appendectomy. Sulu and the landing party are also struck by the absence of non-human animal life. Kirk orders Sandoval to prepare his group to be evacuated to Starbase 27, but Sandoval refuses.
Meanwhile, the colonist Leila Kalomi, shows Spock (in whom she takes a romantic interest) the colonist's secret: plants which spray spores that take up residence in the brain and, in return, provide perfect health and complete well-being. In addition, the plants thrive on Berthold rays. Sulu is soon given a similar treatment, and remarks to Kirk ``Of course we can't remove the colony. It'd be wrong.'' Alarmed by what he sees, Kirk tries to get in touch with Spock. However, it is too late as Spock is currently frolicking with Leila and pointing out cloud formations shaped like dragons from Bering Deria 7.
In the meantime, McCoy is also infected. Not only does he make himself a mint julep, but he also sends up hundreds of plants to the Enterprise. Kirk is beamed up by an infected transporter operator (``Well sure. If you want,'') who is systematically beaming down the crew of the Enterprise. Uhura has short-circuited all communications except ship to surface. Kirk is soon the only one left aboard, and he too is infected by a stray plant left on the bridge. After opening his safe (combination 5231) and looking through its contents, Kirk becomes upset as one side of him wants to beam down and the other side is devoted to the ship. These violent emotions, Kirk soon discovers, drive the spores out.
He subsequently has Spock beam up and proceeds to get him angry by calling him a ``mutinous, disloyal, computerized, half-breed.'' Spock beams up Leila to explain to her what has happened, but the pain of finding out she has lost Spock causes the spores to be driven out from her as well. Leila asks Spock if he has another name, but he only replies ``You couldn't pronounce it.''
Spock prepares a subsonic transmitter which will broadcast over the communicators and provoke those on the planet to irritability and anger. Fights begin to break out between Sulu and DeSalle, then McCoy and Sandoval, and soon the spores are driven out of everyone on the planet. The crew beams back up and the Enterprise is free to continue on its way. Spock is back to his old self, but we find out that on the planet, he was happy for the first time in his life.