The Paranthropus ("tropi" for short) are a large tribe of New Guinea cliff-dwellers who smoke their meat and bury their dead. Simian in many of their physical characteristics, they are normally erect in stance, though happy to drop to all fours at a moment's notice. Australian wool interests see the tropis as a dream come true - workers who can be trained without benefit of paycheck. Newspaperman Douglas Templemore is an idealist - by killing his son (bred by artificial insemination of a female tropi), he hopes to cause a riot in the realm of race relations. Is he a murderer or merely an owner of a pet, which he has "put to sleep?" As he comes up for trial scientific experts file into the witness box; none agreeing on what constitutes a human being. Is man to be defined by his jawbone? By his rational capacity? By his grasp of metaphysics? Or is the judge right when he muses (without a trace of cynicism) that the tropis must be animals because they are not cannibals?