It's New Year's Eve, December 31, 1999, and Washington, D.C., is under siege.

Early in the day, a grisly machine gun attack in the Dupont Circle Metro station leaves dozens dead and the city crippled with fear. A note delivered to the mayor's office pins the massacre on the Digger, a robotlike assassin programmed to wreak havoc on the capital every four hours - until midnight.

Only a ransom of $20 million delivered to the Digger's accomplice - and mastermind - will end the death and terror. But the Digger becomes a far more sinister threat when his accomplice is killed in a freak accident while en route to the money drop. With the ransom note as the single scrap of evidence, Special Agent Margaret Lukas calls upon Parker Kincaid, a retired FBI agent and the top forensic document examiner in the country. Somehow, by midnight, they must find the Digger - before he finds them.