The Enterprise arrives at Omicron Ceti III, hoping to find surviving colonists who have been exposed to deadly berthold rays for three years, and the crew is surprised to find the colony alive and flourishing. Spock beams to the surface and meets Leila Kalomi, a young botanist that he'd worked with previously, who also developed romantic feelings for him. Eager to keep Spock on Omicron Ceti, Leila leads the Vulcan to a secluded section of the planet where a native plant sprays him with their spores. Leila tells Spock that the plant induces feelings of harmony and peace and love, along with a desire to remain on Omicron Ceti III and their paradise.

Jill Ireland (Leila Kalomi)
Frank Overton (Elias Sandoval)
DeForest Kelley (Dr. Leonard H. McCoy)
Grant Woods (Lt. Kelowitz)
George Takei (Lt. Hikaru Sulu)
Nichelle Nichols (Lt. Uhura)
Michael Barrier (Lt. Vincent DeSalle)
Dick Scotter (Painter)
Eddie Paskey (Lt. Lesley)

• The spores thrive on berthold rays, which is why the colonists survived. Anger nullifies the effect of the spores.
  
• After McCoy is overtaken by the spores, he develops an over-exaggerated Southern accent.
  
• Leila first told Spock that she loved him 6 years ago; they apparently worked together on Earth at that time.