Shannyn Sossamon plays artist Mara Sinclair
The Order on IMDB
The film's premise is that there is another way to heaven than adherence to the practices of the Roman Catholic Church. A secular Sin Eater can remove all taint of sin, no matter how foul, from the soul just before death. The purified soul can then ascend into heaven. The Roman Catholic Church, according to the film, considers this heresy.
Heath Ledger plays an unhappy and disillusioned priest, Alex Bernier, whose religious order, the Carolingians, specializes in fighting demons and other hell spawn. Father Dominic, the head of the Carolingians, has died in Rome under suspicious circumstances and Alex leaves the USA to investigate. In Rome, Alex visits the morgue and sees strange markings on Dominic's corpse. After some investigation, he comes across a book that explains the markings as being the sign of a Sin Eater's work. He heads to the Vatican, where an official tells him that Sin Eaters don't exist and that Dominic may not be buried on sacred ground because he had been excommunicated for his beliefs. Alex, moving ever farther from his vocation, defies his superiors and secretly reads a holy service over the body and buries Dominic in the Carolingian cemetery (the service takes place off screen but is referred to later).
Thomas Garrett, another Carolingian (there seem to have only been a total of three including Dominic), arrives in Rome to help investigate Dominic's death.
Early in the film we meet Mara Sinclair, an artist Alex once exorcised, who has escaped from a mental hospital and come to Alex at his church in the USA because she has a feeling that something terrible is going to happen to him. The police come looking for her, but Alex lies and denies that he's seen her and through this exchange we learn that Mara was in the hospital because she had tried to kill Alex during the exorcism. Mara goes to Rome with Alex after promising that she won't try to kill him again.
Cardinal Driscoll (Peter Weller), who is introduced at the beginning of the film and who is tipped to be the next Pope, arrives in Rome from the USA and gives Alex a special dagger. According to a fragment of parchment Alex and Thomas find among Dominic's books, the dagger is to be plunged into the Sin Eater while reciting a text in Aramaic. Alex and Thomas take these instructions to mean that the dagger and incantation will kill the Sin Eater and they begin hunting for the Sin Eater and the remainder of the parchment instructions.
Thomas leads Alex to a nightclub where they are taken to the underground base of operations of a masked man called Chirac, the 'Black Pope.' The Black Pope owes a favor to Thomas and Alex asks where to find the Sin Eater. The Black Pope then hangs three people and tells Alex to ask his question of the dying men who can see what the living cannot. One of the dying tells Alex a riddle that leads to a rendezvous with the Sin Eater.
On the way out of the Black Pope's headquarters, demons attack and injure Thomas, but Alex saves him and gets him to a hospital.
Alex leaves Thomas in the hospital and meets the Sin Eater, William Eden, at St. Peter's Cathedral who explains that he has been a Sin Eater for centuries, taking over for an earlier Sin Eater (a Carolingian priest) who ate the sins of Eden's brother. Eden is very charismatic and talks with Alex about the priest's desires, and Alex admits he wants Mara. He then goes and presumably tells Mara this, and they make love. Afterward, Alex leaves Mara asleep and goes to Eden, who tells Alex that he is tired and ready to die and asks Alex to take his place. Alex has the dagger with him, but is curious and so doesn't use it to kill Eden. Instead, he assists Eden with a sin eating ritual. But in the end, Alex refuses Eden's offer because he has decided to leave the priesthood to be with Mara.
Later Alex returns to their lodgings and finds Mara near death, an apparent suicide. In actuality, Eden slit her wrists and left her for Alex to find. Mara is beyond medical help and Alex quickly performs the sin eating ritual so that she can go to heaven. After absorbing Mara's sins, though, Alex sees that there is no sin of suicide on Mara's conscience and realizes Eden's deception. Alex goes after Eden to kill him.
The rationale that leads Alex to perform the sin eating ritual instead of giving Mara Roman Catholic Last Rites is that Alex has already made the decision to leave the priesthood to be with Mara and he has broken his vows of obedience and of sexual abstinence. He therefore considers himself ineligible to offer Mara Last Rites.
Meanwhile, the injured Thomas is out of the hospital and goes to see the Black Pope who reveals himself to be Cardinal Driscoll. Driscoll shows Thomas the second half of the parchment which instead of being instructions on how to kill a Sin Eater is actually instructions on how to become a Sin Eater. The entirety of Alex's and Mara's lives have been a plot among Dominic, Eden and Driscoll to entrap Alex. Eden wants to die, Driscoll wants to be Pope and Dominic wanted the financial resources to pursue arcane knowledge.
Driscoll prevents Thomas from leaving to warn Alex.
Alex cannot find Eden and returns to the Black Pope to learn where Eden is. The Black Pope (face hidden) tells Alex again to ask the dying. Alex recognizes that Thomas is the man being hanged, and frees him using a pistol. However, Thomas's throat is too injured by the noose to tell Alex the truth of the parchment.
Alex finds Eden and stabs him with the dagger while reciting the incantation. He quickly realizes what is actually happening but it is too late, Eden's powers are transferred to Alex, and Eden, happy to be free of his burden of the sins of others, dies. In the mean time, St. Peter's Basilica in Rome crumbles around them.
Thomas, who arrives at the scene too late to prevent the transfer, vows he will find a way to save Alex, even if it means killing him.
Alex informs the church about Driscoll's activities and Driscoll is ruined. Driscoll then decides to kill himself and calls on the Sin Eater, now Alex, to remove his sins. Driscoll slits his wrists and when he is near death, Alex tells him that he knows that Eden and Driscoll caused Mara's death. Alex does not eat Driscoll's sins but forces them down Driscoll's throat. Driscoll dies a painful death and presumably goes straight to hell.
The Sin Eater William Eden used his power to accumulate wealth. The Sin Eater Alex Bernier decides to act as a power for good, saving only those who deserve it and allowing evildoers to die in sin.
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