After Irish-American brothers Connor McManus (Sean Patrick Flanery) and Murphy MacManus (Norman Reedus) cheat death with the Russian mob a day after St. Patrick's Day, the two decide to voluntarily stay overnight at a police station, where they receive what appears to be a message from God to rid Boston of all evil. Connor and Murphy, with the aid of their friend Rocco (David Della Rocco), set out on numerous campaigns to rid Boston of lowlifes and mafiosos, leaving behind a path of destruction. Meanwhile, F.B.I. Agent Paul Smecker (Willem Dafoe) is assigned the task of investigating the aftermath of each mission conducted by the MacManus brothers and Rocco. The more pieces Smecker fits into the puzzle, the more he asks himself: Should he continue his job and possibly arrest the trio, or join them in their mission to destroy evil?