Alex (HEATH LEDGER) is a NYC priest who practices a rare form of Catholicism that's left the church, represented by Cardinal Driscoll (PETER WELLER), confused about how to deal with him. When he hears that the head of his order, Father Dominic (FRANCESCO CARNELUTTI), has died, Alex sets off for Rome.
He asks former priest and friend Thomas (MARK ADDY) to meet him there and brings along Mara (SHANNYN SOSSAMON), a woman who's escaped from a mental institution. She was there due to previously having tried to kill him around the time he performed an exorcism on her, but he's nevertheless attracted to her, a point that only exacerbates his confused state of mind.
Once in Rome, Alex begins to investigate Dominic's death and doesn't believe he committed suicide as others state. As he and Thomas dig deeper into the matter, they soon come across William Eden (BENNO FÜRMANN). He's a mysterious, immortal figure known as a Sin Eater who takes it upon himself to absolve those who've been shunned by the Church.
As they uncover additional unsettling facts, Alex soon realizes that they've stumbled upon a conspiracy in the highest levels of the Catholic church that involve him in ways he never could have expected.