Stardate 3417.3: The Enterprise approaches Omicron Ceti 3 to investigate an agricultural colony that has been unwittingly exposed to Berthold radiation. They're unable to establish contact with the colony and Spock warns that the colony, under their leader Elias Sandoval, couldn't have survived for three years. He notes that they can handle limited exposure and Kirk orders down a landing party. Kirk, Spock, McCoy, Sulu, and two other men arrive on the planet and discover the colonists alive and well.
Sandoval and the others appear perfectly healthy and rather unconcerned. Kirk proposes to look around and Sandoval introduces Leila Kalomi, a biologist who has met Spock before. The landing party spreads out and notices that there are no animals, while McCoy confirms all the colonists are in perfect health. Sandoval boasts that the soil is perfect for growing anything, but it's clear that they are only growing enough to feed themselves. McCoy conducts tests on Sandoval and informs Kirk that he received an appendectomy… but his appendix has grown back.
Leila visits Spock as he scans for any other life and comments on his lack of expression. She offers to show him the answer to his questions and Spock reluctantly agrees. Meanwhile, Kirk informs Sandoval that he's been ordered by Starfleet to evacuate the colonists but Sandoval refuses.
Leila leads Spock to a strange alien flower-growth that she promises will guarantee peace and happiness. The flower sprays him with spores and he writhes in agony for a minute, then relaxes and admits that he loves her.
Kirk meets with the remaining landing party but Spock and another man, DeSalle, are missing. Kirk calls Spock on the communicator but he's casually dismissive of the captain as he's busy holding Leila. Kirk leaves McCoy to organize the evacuation while he and the others go to find Spock. DeSalle returns to the beam up point… to show the doctor some flowers.
Kirk finds Spock climbing a tree with Leila. After a bit of convincing, Spock takes them to the flowers that spray spores on the other two landing men. Kirk refuses to accept the situation and goes to the beam up point where he finds a much more relaxed McCoy beaming up the flowers to the ship. Kirk goes up and finds that Uhura has sabotaged all but local communications, while the rest of the crew are lined up to beam down to the colony.
Kirk tries to get McCoy to help without luck then comes down to the planet to try to get through to Spock and Sandoval. They explain that the plants drifted through space until they arrived on the planet. They thrive on Berthold rays and establish a symbiotic relationship with the host, providing health and peace of mind. Kirk returns to the ship but by the time he can make a log everyone else has left. A plant sprays him with the spores and he comes under the same relaxing effect. He prepares to beam down but at the last minute is overcome by sudden rage. He snaps out of the spores' effects and realizes that violent emotions and needs--anger--negate the spores.
Kirk then calls down to Spock, saying he needs him to come up to the ship to get some equipment that they'll need since they won't be able to come back to the ship. Once Spock is aboard, Kirk relentlessly insults him until Spock goes berserk and almost kills the captain. Spock manages to snap out of it, overcoming his anger along with the effect of the spores. Kirk suggests they put together a subsonic transmitter and broadcast it over the communicators.
A worried Leila calls up to find out what Spock is doing and he manages to cover up the fact he's free, and agrees to beam her up. She quickly realizes he's back to normal and bursts into tears, realizing she's lost him again, and the emotions free her from the spores as well.
Spock and Kirk hook up the subsonic transmitter, causing a sound that will irritate everyone to hear it. On the planet below, the crew and colonists start fighting among themselves. Sandoval says they want to be located to do actual work and develop a thriving colony.
As the Enterprise leaves orbit, Kirk notes that this time they've walked out of Paradise on their own, and Spock is forced to concede that for the first time in his life… he was happy.