Paul Kersey, played by Charles Bronson, arrives in Brooklyn to reconnect with his old friend from the Korean War, Charley. Tragedy strikes when Charley falls victim to a gang attack in his apartment. Paul witnesses the horrifying moment when Charley dies in his arms, just as the police respond to the disturbance. Misunderstanding the situation, they arrest Paul, thinking he is the perpetrator. During his encounter at the police station, Chief Richard Shriker, portrayed by Ed Lauter, recognizes Paul as "Mr. Vigilante". Following tensions between the two, Paul is thrown into a holding cell where he meets Manny Fraker, the gang leader responsible for Charley's death, played by Gavan O'Herlihy.
Once released on bail, Manny makes a chilling statement to Paul: > "I'll tell you what, I'm gonna kill a little old lady, just for you. Catch it on the six o'clock news." Back on the streets, Manny resumes his role as the infamous leader of his gang, committing further acts of violence as the city's crime rate skyrockets. Recognizing the severity of the situation, Shriker approaches Paul with a controversial deal: Paul is free to enact his own brand of vigilante justice, but he must report any gang activities to the police to facilitate arrests.
To track down the gang, Paul moves into Charley's apartment in a decrepit building, now home to terrified elderly residents including Bennett Cross, a World War II veteran portrayed by Martin Balsam, and Mr. and Mrs. Kaprov, an elderly Jewish couple. After facing harassment, Kersey decides to take matters into his own hands. He cleverly uses a vehicle as bait and confronts two gang members attempting a break-in. Boldly declaring, "It's my car!", Paul finds himself in a deadly confrontation where he draws a revolver and eliminates the threats.
As the situation escalates, Kersey repeatedly tries to protect Maria, a character portrayed by an unlisted actress whose fate takes a tragic turn when she is attacked by Fraker’s gang, resulting in severe repercussions. Driven by vengeance, Paul orders a powerful .475 Wildey Magnum and rigorously prepares himself for the looming battle. The tension mounts as Paul begins to eliminate gang members one by one, leading to a significant rise in neighborhood admiration for his actions.
In his quest for justice, a subplot unfolds involving a budding relationship with public defender Kathryn Davis, played by Deborah Raffin. As their bond strengthens, Paul struggles with his feelings over potential intimacy, particularly after an attempt to enjoy a dinner date spirals into chaos when Fraker targets them, leading to a horrific car accident that claims Kathryn's life.
With the stakes higher than ever, the police, now alarmed, place Paul under protective custody. However, when his friend Bennett, whose taxi shop is later destroyed, decides to take matters into his own hands with Charley's machine gun, he quickly finds himself in dire straits. Paul learns of Bennett’s plight and quickly escapes to gather weapons alongside Rodriguez, a resident played by an unlisted actor.
As the plot reaches its thrilling climax, Paul and Shriker rush to confront an assembly of gang members, employing a Browning M1919 A4 machine gun to devastating effect until they find themselves out of ammunition. The action intensifies as the two men fight valiantly against the criminals, eventually culminating in a fierce showdown where Paul faces off against Manny Fraker. In the stunning finale, Paul dispatches Fraker using a 66mm LAW rocket launcher, and as police sirens echo in the distance, Kersey shares a moment of gratitude with Shriker before making his escape, leaving the remnants of Fraker's gang to retreat in defeat.
