The sequel to the award-winning Oxygen. Courage and self-sacrifice have finally brought the crew of Ares 10 safely to the planet Mars. They enthusiastically begin the mission they were sent to perform. Though working under dangerous conditions and experiencing strained relationships, the astronauts are encouraged by fascinating discoveries that seem to point to the presence of microscopic life on the planet. But their work is disrupted when frightening sabotage, like that of the trip to Mars, begins again. Is the "fifth man" alien or human?