A colony planet bathed in deadly Berthold rays, where alien spores keep the settlers alive and blissfully content.
Omicron Ceti III is home to an agricultural colony of Earth settlers, among them Leila Kalomi and their leader Elias Sandoval. The planet is exposed to Berthold rays, a radiation that should be fatal, yet the colonists are found in perfect health in This Side of Paradise.
The spores of a native plant protect them, and also produce a state of contentment that spreads to the Enterprise crew, including Spock, who finds happiness with Leila. Kirk discovers that strong emotion, especially anger, destroys the spores, and the colonists are relocated once they are freed.