The USS Enterprise, led by Captain Kirk, First Officer Spock, and Chief Medical Officer Leonard McCoy, is directed to a Federation colony on Omicron Ceti III[3]. Upon arrival, they are surprised to find the colonists alive and well, despite the planet's exposure to deadly Berthold rays[3]. Elias Sandoval, the colony leader, and Leila Kalomi, a botanist who once loved Spock, welcome the crew[3]. The colonists appear healthy, and Kirk insists on their evacuation due to the Berthold rays, but Sandoval resists[3].


Kalomi takes Spock to a field of peculiar flowers, which release spores that affect the crew, causing them to disregard their duties and experience perfect health[3]. Only Kirk remains unaffected and returns to the ship, where he discovers that Lt. Uhura has disrupted communication with Starfleet[3]. Eventually, Kirk is also exposed to the spores but overcomes their effects by experiencing violent emotions, which destroy the spores[3].


Kirk lures Spock back to the Enterprise and provokes him into attacking, which frees Spock from the spores' influence[3]. Kalomi's heartbreak upon finding Spock unaffected also liberates her from the spores[3]. Kirk and Spock use an irritating subsonic frequency to trigger fights among the colonists and crew, cleansing them of the spores[3]. Sandoval finally agrees to evacuate the colony[3].


As the Enterprise departs with the colonists, Kirk asks Spock about his time on the planet, and Spock admits that it was the first time he felt happiness[3].