Plot

A young priest is sent to Rome to investigate the troubling death of the head of his order.

Alex Bernier (Ledger) is a member of an arcane order of priests known as Carolingians. When the head of the order dies, Alex is sent to Rome to investigate mysterious circumstances surrounding the death. The body bears strange marks on the chest which may or may not be the sign of a Sin Eater (Furmann), a renegade who offers absolution, last rites and therefore a path to heaven outside the jurisdiction of the church. Alex enlists the aid of his old comrade Father Thomas (Addy) and of a troubled artist (Sossamon) upon whom he once performed an exorcism. He soon finds himself plunged into a mystery only to find himself at the heart of it.—Brian Helgeland

Alex Bernier (Heath Ledger) is a member of an arcane order of priests known as Carolingians. When the head of the Order dies, Alex is sent to Rome by a priest named Driscoll (Peter Weller) to investigate the mysterious circumstances surrounding the death. The body bears strange marks on the chest which may or may not be the sign of a Sin Eater (Benno Fürmann). Alex begins to research this being and discovers that he is an entity existing outside the confines of time (the current sin-eater was born during the Renaissance) who offers absolution, last rites and therefore a path to Heaven outside the jurisdiction of the Church for those the church deems unqualified for the traditional last rites.

Alex enlists the aid of his old comrade Father Thomas Garrett (Mark Addy) and of a troubled artist (Shannyn Sossamon) upon whom he once performed an exorcism. Thomas seeks the counsel of a mysterious hooded figure, apparently the head of some pagan cult existing, literally, in the Roman underworld. They are told how to find the "other" (the sin-eater), who eventually contacts Alex at St. Peter's Cathedral. William Eden (Benno Fürmann), the sin-eater, introduces Alex to his world, explains how he became the sin-eater, demonstrates the rite for him, all in hopes that Alex will take his place and become the next sin-eater. Alex, meanwhile, realizes that his love for Mara can no longer be denied. He leaves the priesthood for her, and rejects Eden's offer.

Not to be denied, Eden murders Mara, removing the only obstacle to Alex assuming the role. Alex, knowing the Mara was murdered by Eden, but acting on incomplete information found on an ancient parchment, decides to kill Eden to rid the world of the sin-eater, but in doing so completes the ritual to become the sin-eater. The movie ends with Alex answering the call of Driscoll. Revealed to be the head of the pagan cult and disgraced at the Vatican, Driscoll decides to commit suicide and calls for the sin-eater to provide absolution. He is surprised when it is Alex and not Eden who arrives, but is further surprised when at the climax of the sin-eating ritual Alex forces Driscoll to consume his own sin, sending him to eternal damnation. The movie concludes as Alex narrates, telling the audience that he walks the earth as the sin-eater, giving absolution to those that deserve it, and denying it to those he deems unworthy.