This Side of Paradise

The Enterprise expects to find the colony on Omicron Ceti III wiped out by deadly Berthold rays, but the settlers led by Elias Sandoval are in perfect health. Among them is botanist Leila Kalomi, who loved Spock years before. She leads him to strange plants whose spores protect the colonists, and Spock is flooded with contentment and able, for once, to return her feelings. The spores spread through the crew, who abandon the ship for the peaceful life below until Kirk is left almost alone. Kirk finds that strong emotion breaks their hold, and deliberately goads Spock into a furious fight, which frees him. Together they broadcast a signal that makes the rest of the crew and colonists irritable enough to recover. Sandoval admits his colony achieved nothing. Spock parts from Leila, reflecting that for the first time he had been happy.