Jim and Elly Mae are exploring the planet Zimmorrah, home of their super-cat friends, the Star Ka'ats. Suddenly Elly Mae senses that someone is hurt and crying for help. She cannont send her thoughts to this strange being, as she can to a Ka'at, but she can FEEL its desperate need.

Searching for the source of the cry, Jim and Elly Mae are drawn to a remote island enclosed in an enormous glass dome. Inside live semihuman plant people who are threatened by evil creatures who look like both a spider and a crab.

The children and their Ka'at friends must fight off the crab/spider monsters and prevent them from destroying the plant people!