The Moon is passing within range of a habitable planet the Alpha staff have code-named 'Retha'—an anagram of 'Earth'. A six-man team is dispatched for a preliminary survey. After several hours, radio contact is lost and Eagle Six fails to return on schedule. Flight telemetry shows all ship's systems operating normally, and a worried orders it flown back to by remote control. He, Victor Bergman and Helena Russell go to meet the mystery vessel. They can find no one aboard...until Helena spies a primitive stone axe and a hairy hand nearly hidden behind a seat in the passenger module.