Spores on a colony planet bring peace and contentment, and even Spock falls under their spell.
The Enterprise expects to find the colony on Omicron Ceti III wiped out by lethal Berthold rays, yet colony leader Elias Sandoval and his settlers are in perfect health after three years, without even their livestock. Spock meets botanist Leila Kalomi, who loved him on Earth six years earlier, and she leads him to plants whose spores cover him. Spock declares his love for her, and the spores, which shield their hosts and keep them content, soon spread to the landing party and then to the crew, who abandon the ship for the planet.
Kirk is infected too, but his anguish at leaving his command burns the spores away, revealing that strong emotion destroys them. He goads Spock with insults until anger frees him, then the two broadcast an irritating subsonic signal to the surface that breaks the spell on everyone. D. C. Fontana wrote it, and Spock admits that for once in his life he was happy.