Paul Kersey, portrayed by Charles Bronson, and his wife Joanna, played by Hope Lange, embark on a vacation in the paradise of Hawaii. Their journey takes a dark turn when they return to New York City, where Paul leads a life as an architect. During a routine grocery shopping trip at D'Agostino's, a harrowing incident unfolds as a trio of hooligans, one of whom is played by Jeff Goldblum, cause chaos in the supermarket. The situation escalates when these delinquents follow Joanna to their apartment, breaking in and wreaking havoc, searching for money but only finding a meager $7. Tragically, they assault both Joanna and their daughter, Carol Anne.
Paul's world shatters when he receives a call from his son-in-law, Jack Toby, informing him that Joanna and Carol are in the hospital. After a torturous wait, Paul learns from a doctor that although Carol is physically unharmed, having been sedated, Joanna has succumbed to her injuries. The police inform the devastated Paul that the chances of catching the criminals are slim, especially since Joanna is gone and Carol remains in a traumatized state, unable to recount what happened to her.
One night, filled with frustration and sorrow, Paul takes a walk and narrowly escapes a robbery when a mugger attempts to hold him up. Armed only with a sock filled with quarters, he manages to evade danger. The following day, his boss sends him on an extended business trip to Tucson, Arizona, where he meets Ames Jainchill, portrayed by Stuart Margolin, who shows him the ropes. Paul witnesses a mock gunfight at Old Tucson and impresses Ames with his shooting skills, after which Ames discreetly gifts him a nickel-plated .32 Colt Police Positive revolver.
Returning to New York, Paul confronts the reality of his daughter’s condition and the weight of his wife’s death. When he opens the suitcase with the revolver inside, he feels an unexpected pull towards vigilantism. That night, while out for a walk, he encounters a mugger named Thomas Leroy Marston and, in a moment of primal instinct, shoots him, marking the beginning of his transformation into a vigilante.
As his quest for justice continues, Paul targets anyone he believes threatens the innocent, leading to a spree of violent confrontations. His reputation as a vigilante grows, drawing the attention of police Lt. Frank Ochoa, played by Vincent Gardenia. Ochoa’s investigation leads him closer to Paul, but the public supports the mysterious figure cleaning up the city.
Compelled by loss and rage, Paul ultimately commits to his new life's mission, while his personal life deteriorates as he places Carol in a mental institution, unable to bear her catatonic state. The ongoing conflict between Paul and law enforcement escalates, culminating in a confrontation that requires Paul to leave New York to avoid arrest.
In the film's thrilling conclusion, after a week’s respite, Paul arrives at Chicago Union Station, only to find himself embroiled in chaos once again as he intervenes to rescue a woman from a group of hoodlums. With a smile and a pointed finger mimicking a gun, he signals that his fight for justice will undoubtedly continue, leaving viewers to ponder the complexities of vengeance and morality.
