A young boy named Sean Donovan lives with his mother and abusive alcoholic father. A carjacker attacks his family one night, which results in his mother’s murder. Afterward, his father Terry moves them to a nicer neighborhood.
Years later, Sean is an excellent student but has become unsocial, and he sees abuse in everyday arguments. He trains as a boxer and carries weapons at night as a vigilante. He kills an assaulter in an attempted mugging, and an eyewitness account garners the attention of Teresa Ames, who had recently been promoted to the homicide division of the NYPD. She investigates Sean, who frequents the police station as he searches for information on his mother’s killer. Teresa befriends him and learns about his life, such as the fact that he speaks fluent Chinese after he angrily berates rude staff in a Chinese restaurant. Sean defends a young woman being violently abused by her pimp and strikes him with a baton before shooting him.
While riding the train, Sean encounters a homeless man verbally harassing a Chinese family and the passengers of the train. He tells the family to leave the car before putting on face paint and brutally beating the man with brass knuckles. Teresa and her partner, who happen to be on the train, investigate, but Sean eludes them. Learning that the perpetrator spoke fluent Chinese, she suspects Sean. Despite her supervisor’s warning not to investigate Sean, she persists. Finally, the retired supervisor tells Teresa that the young Sean was able to clearly identify his mother’s murderer from a photo book, but Terry convinced him to withdraw his statement. The murderer’s identity is identified as Larry Childs, a contract killer whom Teresa arrested six months ago but managed to get a two year sentence through a plea bargain.
As Sean walks down the street, he sees scenes of domestic violence that apparently revert to gentle arguments after he passes by. During a school party, Sean has an episodic memory recall where he remembers his mother’s murderer call his father by his old boxing nickname. Enraged by the memory, he savagely beats a fellow student who has been harassing a female friend. Teresa finds a picture of Terry and the murderer and realizes that they knew each other before the attack. Sean assumes that Terry staged the attack in order to get his mother’s life insurance money. Sean confronts his father, but Terry adamantly denies this. Convinced of his father’s guilt, Sean shoots and kills Terry. Teresa tracks down Sean, who states someone broke into their home and killed his father, and she disposes of the murder weapon.
Sometime later, Sean sends a letter to Larry, who is currently serving his two year sentence. In the letter Sean expresses his forgiveness to Larry but pleads with him to reveal the truth of his mother’s murder: was his father involved? He puts an empty envelope, a red stamp, and a black stamp together with the letter, and asks Larry to reply to him with a black stamp for his father’s guilt or a red stamp for his father’s innocence. The stamp Larry uses is poisoned with Tricelaron and kills him. When the letter arrives, Sean stares at the red stamp on it.