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.