The film takes place in the Flats of Boston, a racially segregated Irish Catholic neighborhood where one of three young boys is abducted and assaulted by two men. Ironically, one of the pedophiles lures Dave Boyle by impersonating a police officer who catches the boys writing their names in wet sidewalk cement. Dave’s name remains incomplete below Sean’s and Jimmy’s in the fresh repair, and the theme of the movie is clearly established insofar as Dave’s life will always be incomplete and unhealed after all his innocence is lost during four days of captivity in a remote farmhouse, deep in the woods.

The familiar theme of lost innocence is coupled with the omniscience of the Roman Catholic Church in this small town within a large city. One of Dave’s abductors with white hair and a kind, smiling face turns to confront the frightened boy in the back seat of the car and flashes a large gold signet ring with a cross embossed on it. Years later, adult Jimmy sports a Celtic cross tattooed from the back of his neck to his waist. And in the early morning hours of the night of his nineteen-year-old daughter’s murder, a day on which she had planned to wed, the family prepares for their second daughter’s first holy communion, an elaborate ceremony of important religious and social significance promising redemption and hope. Tragically, it becomes a rite of passage which ends with the discovery of cherished Katie’s mangled body in a public park. At this juncture, vengeance supplants religious consolation and a great tragedy is launched.

The tragic elements are fomented by overlaps, relationships and coincidences in the lives of the three boys that motivate behaviors that may or may not be out of their control. The relationship between Jimmy’s daughter Katie and her boy friend Brendan Harris recalls Romeo and Juliet when Sean, who grows up to become a detective, uncovers their plans to elope to Las Vegas. The match was further cursed by Jimmy’s unexplained hatred, reminiscent of the Montagues and the Capulets, of the boy’s family, a condition which imposed lies and subterfuge on the young lovers. Another link to classic tragedy is Brendan’s parentage and the fact that Jimmy had murdered Just Ray Harris years before. Thus through centuries echoes Hamlet’s dilemma of avenging a parent’s death, a killing performed by a potential in-law. These two situations are not identical to their literary predecessors, but provide notably similar circumstances, enough to be intriguing especially when updated to contemporary Boston in the un-Shakespearean genre of film.

The central tragic hero is Dave whose tragic flaw is his shame. Shame hangs on him like a bulky overcoat in the way he walks, the softness of his voice, his listless demeanor, and his aimlessness. Unlike traditional tragic heroes, Dave’s flaw originates from without his being, from sociological conditions beyond his control, from the Irish Catholicism of his small community and its unwillingness to openly confront issues of human sexuality and its anomalies. When Dave escapes his captors and returns home, the camera lingers on his family’s townhouse with one light on in an upstairs bedroom. Poignantly, a woman walks to the window and pulls the shade, leaving the outside of the house in darkness. Her gesture suggests that Dave’s molestation is a private, secretive matter contained within the confines of the family home. His family’s cloistered reaction deprives Dave of healthy psychological healing and he carries the burden of his humiliation for the rest of his days, haunted by flashbacks and unexpressed rage that crescendos to a violent act implicating him in Katie’s murder. Like Desdemona, he is betrayed by his loving mate and dies innocent of the crime for which he is accused but untried, a victim of circumstances and his culture’s blindness to his psychic wounds.

If Dave is a study in shame, Jimmy Marcus is a study in evil. Defying Eastwood’s mono-dimensional villains such as the lunatic in Dirty Harry, Jimmy evolves, morphing from loving husband and father into a murdering monster with a murky past which reveals itself gradually. Only Sergeant Powers, an outsider and an African-American, senses Jimmy’s criminality and notes his defensively defiant aura, one which can only be acquired in a penitentiary. Jimmy’s misdeeds include serving time for armed robbery, killing Just Ray Harris, the father of his daughter’s lover, and murdering Dave in an act of furious vengeance based on falsehoods. Simultaneously he is a parent who smiles proudly at his daughter’s first communion in a scene that recalls the baptism in The Godfather. While babies are sanctified, violence erupts outside of the church’s protective walls.

Jimmy’s complexity as a villain derives from the fact that he is a family man who ostensibly loves his wife and daughters, and grieves deeply over Katie’s death. Two scenes, one in a morgue, one in the mortician’s basement, evoke pity and fear at the depth of his despair and disbelief as he stares at his daughter’s remains. Though he becomes a ruthless killer, he is not a villain lacking in humanity.

As for the women who marry the three grown boys, Sean’s wife has left him, and Jimmy and Dave are married to cousins Annabeth and Celestine. Dave’s wife Celestine is meek, quiet and supportive, yet doubts Dave’s alibi of a mugging when he returns home at three a.m., the night of Katie’s death, covered with blood. She wants to believe in his innocence, but too much coincidental evidence informs her otherwise. Witlessly, she confides to Jimmy and in doing so her goodness betrays her. Her belief in a clear conscience and doing her duty guarantee her husband’s slaying, and she is left with his death on her, ironically, innocent hands.

Of the three women, Annabeth is the most dangerous because she longs to control. Upon arriving at the morgue to identify Katie, she tells Jimmy that it is all a mistake the authorities must correct. She orchestrates her family’s life the same way by issuing orders and schedules, and she functions in the film as a foil to her meek cousin Celestine. The film culminates in a chilling bedroom scene wherein Annabeth urges Jimmy to conceal his crimes, assert his courage, and carry on family life despite his track record as a killer. He submits and they seal their pact by making love. Lady Macbeth prevails over her doubtful but yielding mate.

The community of the Flats and its intertwined, tangled lives is left in moral turmoil and Eastwood chooses to leave ends unraveled by closing the film the same way it opened with a long low shot of the Mystic River. Before doing so, Sean signals Jimmy with his forefinger and thumb poised as a shotgun and thereby suggests that he will try to do justice for Dave, his childhood friend. This is the only suggestion of restored order, the traditional ending for a great tragedy.