John Mars is the oldest astronaut in the fleet and the commander of Challenger II. His latest assignment from NASA is a top-secret shuttle mission to the moon to investigate a mysterious signal. What he finds is an alien robot that murders his crew and destroys the source of that signal-a deep-space probe. When John returns to Earth without his crew, the probe, or any evidence of what really happened on his mission, NASA forces him into retirement.

Humiliated and disgraced, John takes a job as a pilot for a commercial airline, unaware that his experience on the moon has marked him more...