Alone

Massachusetts State Police Trooper Bobby Dodge, a trained sniper on the Special Tactics and Operations (STOP) team, responds to a late-night domestic barricade in Boston. Catherine Gagnon has reported that her husband Jimmy is threatening her and their four-year-old son, Nathan, with a gun, and neighbors have reported shots. From a building across the street, Bobby observes Jimmy pointing a pistol at Catherine. Believing her life is in immediate danger, Bobby shoots and kills him. Catherine appears to thank Bobby, but the shooting subjects him to a district attorney's investigation, intense scrutiny, and a separate clerk-magistrate petition brought by Jimmy's mother, Maryanne. Jimmy's father, powerful superior court judge James Gagnon, pressures Bobby to claim falsely that Catherine deliberately provoked her husband, offering to end the legal action if Bobby cooperates.

Catherine's history complicates every interpretation of the incident. In 1980, when she was twelve, Richard Umbrio abducted her and confined her underground for twenty-eight days before she was rescued and identified him to authorities. The imprisonment left her fearful, isolated, and vulnerable long after the trial. Years later, a call to a suicide hotline put her in contact with the volunteer who would become her husband, Jimmy. He secretly located her afterward using the information she had disclosed during the crisis call. Their marriage deteriorated into drinking, infidelity, violence, and struggle over their desperately ill son. Nathan has recurrent pancreatitis, poor growth, broken bones, and restrictive diets without a convincing diagnosis. James and Maryanne accuse Catherine of harming the boy to attract attention and seek custody, while she believes they intend to control Nathan and the family's money. Her former lover and Nathan's pediatrician, Dr. Tony Rocco, disputes the assumption that Catherine is responsible for the symptoms, although he cannot initially identify their cause.

Bobby initially resists Catherine's efforts to enlist him but becomes involved in the custody dispute and the investigation. The case affects him personally: his own father was an abusive alcoholic, his mother fled when Bobby was young, and his brother George was stabbed during a confrontation with their father. With psychologist Dr. Elizabeth Lane, Bobby begins to confront the anger, abandonment, and misplaced responsibility that drive his identification with Nathan. His relationship with cellist Susan Abrahms ends, his confidence in his conduct deteriorates, and Boston homicide detective D. D. Warren, a former girlfriend, warns that his private dealings with Catherine threaten his career. Susan also recalls Bobby's previous encounter with the Gagnons at a symphony event, where Catherine asked about the kinds of incidents that prompt tactical-team intervention. Bobby cannot dismiss the possibility that the hostage scene was manipulated.

As the dispute escalates, Rocco is stabbed and mutilated, and nanny Prudence Walker is murdered with her neck broken and her body staged as a suicide in Catherine's townhouse. The killer is the recently paroled Umbrio, who calls himself Mr. Bosu. He has been hired through criminal intermediary Colleen Robinson by a hidden employer, later identified as Judge Gagnon, and is also driven by his longstanding obsession with Catherine. Bosu leaves threatening 'Boo' messages, enters Nathan's room and removes its lightbulbs, and deliberately exploits Catherine's fear of darkness. He kills Assistant District Attorney Rick Copley as another paid assignment and uses evidence from the prosecutor's house to deepen suspicion that Bobby is responsible. He ultimately kills Robinson as well. Meanwhile, investigators find that Jimmy's own gun was not fired during the standoff: the reported shots came from a second pistol belonging to Catherine, hidden in a bedroom bureau. Catherine acknowledges acting to prevent Jimmy from taking Nathan but insists that Jimmy genuinely threatened her and that Bobby saved her life. Bobby is left with a justified perception of imminent danger and serious doubts about how that perception was produced.

Nathan's new geneticist, Dr. Iorfino, makes a crucial discovery using samples Rocco had submitted before his death. Nathan has Fanconi-Bickel syndrome, a rare inherited metabolic disorder that explains his illness and can be managed through specialized treatment and diet, though not cured. Drawing on the diagnosis and evidence that Maryanne's earlier records have disappeared, Bobby suspects James and Maryanne concealed a close biological relationship. When the family confronts one another at the end, Maryanne confirms that she and James are half siblings who met after growing up apart. Catherine alleges that James engineered earlier family deaths, including those of Maryanne's relatives and their first infant, to conceal or preserve the relationship. These additional accusations are not clearly admitted by James or conclusively proved. Nathan's diagnosis, however, exposes a family secret that provides a plausible motive for the judge's determined custody campaign and for his efforts to discredit Catherine.

Umbrio murders Catherine's father, Frank Miller, while abducting Nathan. He demands a ransom from James, who refuses and indicates that Nathan's death would resolve a problem for him. Learning that the grandparents are staying at the Hotel LeRoux, Umbrio takes Nathan there. He kills a hotel employee and investigator Harris Reed while forcing entry into the penthouse, then gravely wounds James and stabs Bobby in the shoulder. Catherine helps Bobby draw Umbrio into an ambush. Bobby shoots the attacker but runs out of ammunition, and Catherine kills Umbrio with a second handgun when he reaches for her throat. Nathan survives and is reunited with her; Bobby is temporarily arrested because police also suspect him of Copley's murder.

In the January epilogue, Bobby has been cleared in the Jimmy shooting and Copley investigation and is preparing to return to police work. He has made progress in therapy and in his relationship with his father, although he still has nightmares. Nathan is healthier on his new diet, and Catherine intends to move with him to Arizona. James remains alive in a coma from which doctors do not expect him to recover, while Maryanne stays devoted to him. No charges are brought against Catherine for Jimmy's shooting. D. D., however, has found two telephone calls from Catherine to Robinson, although no conclusive communications linking James to the broker. Wearing a recording wire, Bobby confronts Catherine with his theory that she used the judge's hired murderer to eliminate Copley and turn Umbrio against James. She neither clearly confirms the theory nor gives an admission that would support charges. Bobby declines her invitation to accompany her and Nathan to Arizona. The family survives and Bobby returns to his profession, but Catherine's ultimate role in directing the violence remains deliberately unresolved.