Spores on Omicron Ceti III protected a colony from deadly radiation but left people blissfully content, and they spread through the Enterprise crew until Kirk found how to break their hold.
The Enterprise expected to find the colony on Omicron Ceti III dead from Berthold rays, but its leader, Elias Sandoval, and his settlers were healthy and happy. Spores from a native plant had protected them and taken away all their ambition. Spock was exposed and found peace and love with botanist Leila Kalomi, and the spores spread to the rest of the crew, who left the ship to join the colony.
Kirk, last aboard, found that strong emotion destroyed the spores. He provoked Spock into rage, freeing him, and the two built a subsonic device that drove the spores out of the whole colony. The settlers were moved to a new world.