"This Side of Paradise" is the twenty-fourth episode of the first season of the American science fiction television series Star Trek. It was first broadcast on March 2, 1967, and was repeated on August 10, 1967. The episode was written by D. C. Fontana and Jerry Sohl (using the pseudonym Nathan Butler), and directed by Ralph Senensky. The title is taken from the poem "Tiare Tahiti" by Rupert Brooke and the novel "This Side of Paradise" by F. Scott Fitzgerald.

In the episode, the USS Enterprise visits a planet where the inhabitants are under the influence of strange plant life.

Enterprise is ordered to a Federation colony on Omicron Ceti III. Shortly after the colony was founded some years before, it was discovered the planet was bathed in Berthold rays, a lethal form of radiation. The colonists are now presumed dead.

Captain Kirk, First Officer Spock, Chief Medical Officer Leonard McCoy, and others beam down to the colony, and are surprised to find the colonists all alive and well. Their leader, Elias Sandoval, welcomes them and explains they only lost communications due to equipment failure. Also present is Leila Kalomi, an acquaintance of Spock's. Kirk orders the landing party to explore the colony. They notice the lack of animal life including livestock brought to the colony. Sandoval and other colonists allow McCoy to examine them. McCoy finds no sign of disease or injury in any of them: even Sandoval, who has had an appendectomy, now has a healthy appendix.

Kalomi offers to show Spock how the colonists have survived, and takes him to a field of strange flowers. The flowers expel spores that cover Spock, after which he professes his love for Kalomi, and blithely disregards orders to begin the evacuation of the colony. The rest of the landing party are also exposed to the spores and, with the exception of Kirk, exhibit the same sort of behavior. It is eventually revealed that the spores, in addition to this tranquilizing effect, also provide perfect health including protection from Berthold rays.

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