Cmdr. Pirx is ordered to lead five astronauts on a space mission to Saturn. But his orders also entail his identifying which one of the crew members is secretly a humanoid. Problems arise when one of the astronauts, Harry Brown, claims that he can help spot the artificial being, which confuses Pirx. A serious accident occurs during the trip and they return to Earth. Now Pirx, who is blamed for almost killing his crew, must clear his name at a tribunal.