movie trailer (apple.com - quicktime)
NOTE: This spoiler was sent in by AYN2578.
The movie starts with priest Alex Bonnier (Ledger) explaining how much his life is weird and how he now sees the world with a different eye.
Flashback to Rome where we follow an old priest riding a bicycle through the streets to what looks like an abandoned tower-like house. The priest enters and pays very little attention to the two strange-looking kids sitting on the floor in the hall, staring blankly at him. He then enters his apartment and starts looking through papers.
Enter Eden, the Sin Eater who tells him it is time to go. The priest utters that he has sinned against Alex and that he is sorry. The Sin Eater eats his sins and he dies (we don't see it happening, just flashes of light from the next room).
Cut to Bonnier performing mass in Latin (he is from the Carolingian Order, Priests that kill demons perform exorcisms). After mass he gets a visit from an important priest (Peter Weller). Weller tells him that the head of his Order is dead and apparently murdered in some very weird way. Bonnier gets emotional and remembers when he was young and Brother Something took him under his wing (we see Bonnier's dead mother on the hospital bead, she slit her own wrists). We now know that Bonnier and another priest (Mark Addy) are the only remaining priests of their order. So Bonnier decides to go seek Addy to help him out with the investigation.
We then see Addy, being lured out of his church in Rome by strange voices. He proceeds to take out his cross and shout in the name of the lord to banish the demons. Then a man pushes him as he reaches the courtyard and starts running. He chases the young man in the streets who manages to get hit by a truck. The man utters the church will not stop the rising of the Black Pope and that it's useless to try before transforming into a demon and dying.
Back to Bonnier, he receives the visit of a strange girl (Shannyn Sossamon) who tells him she needs a place to crash. He asked her how she got out, she tells him she's fine then we hear a pounding at the church door. It's the police asking Bonnier if he seen Sossamon cause she escaped from the mental ward. He says no. The cop says he wanted to warn him because after all she tried to kill him a few years back during a session of exercise. Bonnier corrects the cop that she tried to kill him during a session of exorcism. Back inside he tells the girl he has to leave for the investigation and tells her she can stay at the church while he's away. She wants to come with him and he finally agrees as long as she doesn't try to kill him. Apparently she's a painter who's afraid of painting sunflowers because it makes her hear nightingales (birds) and that's what drove her crazy (or possessed).
Arriving in Rome, Bonnier and the girl go to the murdered priest's apartment and they see the two weird children in the hall, playing and looking freaky. The girl asks what they are and Bonnier answers "orphans" but adds "of who or what, though, I'm not sure". They finally enter the apartment. On a table, Bonnier finds strange markings in Aramaic (the language of Christ) that says "blood in, blood out" and he remembers the story of the Sin Eater ritual. They find a piece of a parchment the brother was apparently looking for. He tells the girl to stay in the apartment, he has to find his friend and start the investigation.
First he goes to the morgue to see the brother's corpse. He finds that the corpse bears a strange scar over the heart but cannot look much further because the nurse will not let him. He pays the nurse and leaves.
Then he goes to the Vatican to see an important official and explains to him the situation. The head priest is not fond of this order of wacko ghost believers and considers them near heretics. He claims that the brother committed suicide and that Sin Eaters do not exist. He also tell Bonnier that the brother was excommunicated not too long ago for his pagan beliefs. Bonnier denies it and asks the head priest to call the counsel to at least allow the body to be properly exhumed and buried in a Christian manner. The head priests refuses and tells him to leave.
Bonnier goes to a library and starts talking priest stuff with the storekeeper who finally agrees to bring him downstairs to look at some forbidden knowledge material. On his way down, the storekeeper warns Bonnier to stay between the lines drawn on the floor. Bonnier asks why because he realizes he's standing outside the lines and suddenly a freaky looking dog on a chain jumps at him menacingly, only to jump back into the darkness in some surreal looking way (like a cerberus guarding the place, or something). He then starts looking at the books and finds information on the Sin Eater (looking at a drawing in the book, he sees the "victim" on a table with the Aramaic markings, and the strange looking scar near the heart).
Bonnier then walks into the hospital at night and steals the corpse of the old priest. With the help of two nuns he buries him properly in the cemetery and fills the hole himself. When he heads back, the wind starts blowing ominously and the leaves start flying all over the place. Then we see the two freaky children who followed him.
The children reveal their demon faces and Bonnier gets thrown to the ground twice. He starts banishing them and they finally vanish in a "cloud of bats". Then Addy arrives and greets Bonnier like nothing ever happened; in fact this demon-killing business looks very dull and boring, like a normal job (they make the action seem so unimportant in this movie).
Bonnier brings Addy back home and he meets Sossamon. They proceed to decipher the piece of parchment and find more about the Sin Eater.
Then Peter Weller pays them a visit to give them a dagger and tells them it is the way to destroy the Sin Eater.
(Now this is where it starts to get blurry for me, so bear with me. The events I will explain up to this point might not be in the right sequence but it all ends up to the finale anyways...)
Bonnier and Addy are at a dead end, they need the second half of the parchment and don't know where to find the Sin Eater. So Addy brings Bonnier to a club of pagan priests where they go underground following a strange looking chick with a distant stare. In the catacombs they witness executions by hanging orchestrated by what seems to be the Black Pope, veiled and speaking in latin. The Black Pope tells them to ask the dying. He orders his followers to hang a person in a black bag. The dying person gives them the name of the Sin Eater.
They leave and on their way up, Addy tells Bonnier he should consider to make out with the girl. Bonnier is confused and torn between his role as a priest and a man with hormones/emotions. Bonnier asks Addy if he ever felt that way, Addy says yes and in the meantime, demons start calling out to Addy. He wants the harassment to stop so he goes to confront the demons. The demons take the guise of the prostitute Addy fell for when he was a young priest and as he starts to banish them nails start flying into him, "crucifying" him on a beam of wood. Bonnier arrives to the rescue, demons disappear and he saves Addy. Bonnier brings him to the hospital.
Bonnier somehow meets with Eden (the Sin Eater) and Eden explains his story, that he is centuries old and that he became the Sin Eater to save the ones who were denied absolution for their sins by the church, to permit them to go to heaven even without the religious consent so to speak. Eden helped with the construction of the St-Peters cathedral. Bonnier has the dagger with him but cannot bear to use it, he seems curious to find out more about this mysterious being. So he follows him around and they talk about how religion is crap and about how much Bonnier would like to be with the girl but he is a priest and he can't, yadda-yadda-yadda. Bonnier assists a Sin Eater ritual, compelled to watch and intrigued once more.
Bonnier goes back to the apartment and makes love to the girl, giving in to his human side. We see that she has freaky tattoos and scars on her arms (suicide attempt). She is happy, he is happy.
Eden meets with Bonnier and tells him he is tired of being a Sin Eater and proposes him to take his place. Bonnier refuses.
Eden makes the girl go mad with nightingales and sunflower paintings and kills her, making it look like suicide.
Bonnier finds the girl, weeps and performs the Sin Eater ritual in order for the girl not to go to Hell. He feels like God, he likes it. He knows that Eden killed her though so he goes out for revenge.
Meanwhile, Addy comes out of the hospital and starts suspecting something is wrong, so he goes alone to see the Black Pope, who reveals the second part of the parchment. As he goes to warn Bonnier of what is on the 2nd piece, the Black Pope orders his followers to capture him. He then reveals his face, it is Weller. He says that no one will be able to prevent his ascension to Pope status and that he orchestrated everything.
Bonnier cannot find Eden, goes to see the Black Pope, steals a gun, fires a few shots at the followers who back away, asks the Pope where Eden is and he tells Bonnier to ask the dying. Another hanging but this time, Addy is in the black bag. Addy says to go to the St-Peters cathedral, where it all started. Bonnier recognizes his friends voice and shoots the rope, frees his friend and they both escape. Addy is too shaken to be able to tell Bonnier to stay, so Bonnier leaves for St-Peters.
Bonnier stabs Eden in St-Peters in front of the raging crowd and he suddenly absorbs all the sins that Eden had inside. It was all a scam for him to become the Sin Eater. The stabbing allows for the transfer not the destruction.
Bonnier is now very angry and goes back to give Weller his absolution, turns out Bonnier told the church about Weller's pagan activities and he is now excommunicated. For some reason Weller wants to kill himself and Bonnier offers to eat his sins. At the last minute though, he tells Weller that he knows he's somewhat responsible for his girlfriend's death and decides not to save him...
Weller dies a painful death and goes to Hell.
Finally, Bonnier explains off camera that he is now the Sin Eater and that he now decides who goes to heaven and who goes to Hell...