In the summer of 1950, American retirees Judith and Henry Geeth join their Irish friend Gert O’Connell for a holiday at the isolated Scottish coastal estate of another old friend, Margit Baltry. On their arrival, however, the trio find that their visit has been overshadowed by the mysterious disappearance from Margit’s mansion of a member of a traveling carnival during a party several days previously. Much to the dismay of their hostess, Judith and Henry eagerly take up the task of investigating the disappearance themselves, with Gert’s reluctant assistance. The investigation takes a more sinister turn when the self-appointed private eyes learn that the Baltry estate harbors a centuries-old legend about an ancient Druid sect, buried treasure, and a murderous “Beast from the Sky” that resides on an island off the coast. It is inferred by some that this Beast may be responsible for the disappearance of the party guest. According to local legend, the Beast has claimed other lives in the remote past, and it still maintains a jealous vigil over its domain. As bodies begin to turn up and signs pointing to an elaborate murder plot emerge, Judith and Henry enthusiastically embrace their newfound roles as unsolicited assistants to the dubious village police inspector, gathering clues on their own and interviewing the suspects, which include other members of the carnival troupe, residents of a nearby estate, members of Margit’s own household, and two itinerant scientists engaged in research on coastal fishes. All the while, Gert bemoans the apparent forfeiture of the peaceful vacation she had anticipated, and tries to sort out her own inner conflict between the seemingly supernatural aspects of the case and her twentieth-century allegiance to rational thinking.