In the cold, gray city of Boston, two Irish brothers, Connor and Murphy MacManus, lead a mundane life until one night their existence is changed forever when they intervene in a conflict involving a group of Russian mobsters. After defending themselves, they realize they have eliminated these criminals and, in an act of personal justice, decide that it is time to take matters into their own hands. Convinced they are fulfilling divine will, they set out to eliminate the gangsters that plague their city, believing they are purifying the world of scum. With each murder, their notoriety grows, and they become urban legends, known as "the Saints."
Meanwhile, a baffled detective, Paul Smecker, finds himself in the position of solving these homicides. As he delves into the case, Smecker begins to question the legality and morality of the brothers' actions. With a growing game of cat and mouse, the Saints become a moral dilemma for both criminals and authorities.
Amidst bullets, stylized violence, and memorable dialogue, the story raises profound questions about justice, revenge, and loyalty. As tensions rise and the paths of the brothers and Smecker intersect, everyone must confront the decisions they have made and their inevitable consequences.