A murderer is on the loose on the streets of New York City: nicknamed the Doomsday Killer, he's claimed six victims in just two weeks, and the city is terrified. Even worse, the police are mystified: the victims have nothing in common, defying all profiling, and all that connects them is that each received a sick postcard in the mail before they died - a postcard that announced their date of death. In desperation, the FBI assigns the case to maverick agent Will Piper, once the most accomplished serial killing expert in the bureau's history, now on a dissolute spiral to retirement.

Battling his own demons, Will is soon drawn back into a world he both loves and hates, determined to catch the killer whatever it takes. But his search takes him in a direction he could never have predicted, uncovering a shocking secret that has been closely guarded for centuries. A secret that once lay buried in an underground library beneath an 8th Century monastery, but which has now been unearthed - with deadly consequences. A select few defend the secret of the library with their lives - and as Will closes in on the truth, they are determined to stop him, at any cost ...