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 than psychologically: in the battle to save his crew and himself, John Mars was injected with an alien tracking device.

One night, while flying his regular route, John and the occupants of hisplane, including his fiancé Anna, are whisked aboard a gigantic ship and thrown into deep space toward the Andromeda Galaxy-home to an Alien species called "Sels". The Sels live for one thing only, colonization of all worlds in the known universe. And now they've got their sights set on Earth. But in order to decide if Earth is a worthy adversary, the Sels have something special planned for John-a kind of alien vetting program.

John, Anna and the rest of his group arrive on a Sel planet used for wargames, where alien life forms are pitted against one another-and the Sels-in death battles. One by one, John Mars' small group of survivors succumb to the Sels. But a larger problem looms for Mars-even if he can survive the Sel wargames, how could he ever get back home and warn Earth of impending invasion and assimilation into the Sel Empire?