The story takes place in an era when humans have colonized the Solar System but dare not go farther, as the first extra-solar expedition to Barnard's Star failed and the survivors came back as babbling, grotesque, diseased madmen. They spoke of a gigantic planet, populated by ferocious animals and the single city left of the evil "Medusae". The Medusae bear a vague resemblance to jellyfish, but are actually elephant-sized, four-eyed, flying beings with hundreds of tentacles. The Medusae cannot speak and communicate with one another via a microwave code.