Maya Larkin (Winona Ryder) is a young woman who the Catholic church uses as a lay-person when performing exorcisms, since she was once possessed herself.
While having to deal with a particularly nasty possession, she finds that the victim had been writing down a series of seemingly random numbers.
On examining these numbers she cracks the code and comes up with a name: Peter Kelson!
Peter Kerlson (Ben Chaplin) is a successful author dealing with criminology and forensics, and he's been having some very strange dreams recently.
Now Maya has figured out that Peter is going to be the anti-Christ.
She tracks him down and tells him this; to which he responds skeptically but eventually has no other alternative than realize it's true. His future now is that on his 33rd birthday, he is going to "transformed" into the actual embodiment of Satan, and the whole world better get back!