Many scientists believe that if aliens invade Earth they will arrive in the form of microbes riding meteorites as they plunge through the atmosphere and hit the planet's surface. Or perhaps they're already here, hidden in human bodies, gradually transforming their hosts into mindless slaves.|
An infected university professor commits suicide in order to leave a message for an ex-student named Sara Devlin. She returns home to discover that her old friend had been host to an alien parasite. Her attempts to expose the danger take her down a path that leads to a computer called the Crop Circle, research suggesting that human heads have been exploding for thousands of years, and an on-again off-again affair with a professional meteorite hunter who has secrets of his own. Together they battle the government and the alien menace while the future of the human race hangs in the balance.