When a shapeshifting alien disguised as an American astronaut reaches Earth with dire plans for civilization, a freespirited loner troubled by psychic visions is recruited by a secret agency to hunt down the creature, code named Spartan.

The visions plaguing Corbet Tomms leave him with a fear of intimacy and a penchant for meaningless sexual conquests. Aggie Rittenhouse, his no-nonsense FBI handler, challenges his playboy status and pushes him to confront the roots of his emotional solitude.

But the mismatched duo face far worse problems. As they close in on the murderous alien amid extraordinary danger, Corbet realizes that Spartan is at the heart of his most terrifying vision, one that portends global disaster.

Spartan must be found before it locates a hidden, long-lost spaceship and activates a freakish weapon that will forever alter what it means to be human.