An arid pre-warp world threatened by the comet M'hanit, which the Enterprise deflected so that it enriched the planet instead.
Persephone III is the arid planet at the center of Children of the Comet, home to a pre-warp people. A comet is set to strike it and kill all its inhabitants, but a force field around the comet prevents the Enterprise from altering its course.
The Shepherds, who believe the comet is the arbiter of life M'hanit, try to stop interference. Spock nudges the comet past the planet anyway, and its water vapor improves Persephone III's conditions for life.