Paul Kersey (Charles Bronson) has arrived in Brooklyn to visit his longtime Korean war buddy friend Charley, who is attacked by a gang in his apartment. The neighbors hear commotion and call the police. Paul arrives as Charlie collapses dead in his arms. The police arrive and mistakenly arrest Paul for Charley's murder. At the police station, chief Richard Shriker (Ed Lauter) recognizes Paul as "Mr. Vigilante". Shriker lays down the law on Paul, and the two get into a scuffle before Paul is taken to a holding cell. In the same cell Manny Fraker (Gavan O'Herlihy), the leader of the gang who attacked and killed Charlie, is held. Fraker and Paul get in a fight before the police break it up. A few moments later, Fraker is released from the jail after his fellow gang members post his bail. Before walking out, he threatens Paul and states: "I'll tell you what, I'm gonna kill a little old lady, just for you. Catch it on the six o'clock news."
Manny arrives back on his gang turf to take his place as leader, slashing fellow gangmember Hector (David Crean), possibly for betrayal (and later he and his gang kill either a resident or a member of a rival gang when he accidentally enters Manny's turf). Manny's gang is also responsible for the recent thefts in the apartment where Charley was staying.
The police receive daily reports about the increased rate of crime. Shriker arranges to talk with Paul and offers him a deal: Paul can kill all the punks he wants to kill, but has to inform Shriker of any gang activity he hears about so the police can get a bust and make the news. Paul agrees, and moves into Charley's apartment in a decaying building in the middle of a bombed out gang war zone. The building is populated by a group of elderly tenants who are absolutely terrified by Fraker and his gang. Kersey befriends some of them: Bennett Cross (Martin Balsam), a World War II veteran and Charley's good buddy; Mr. and Mrs. Kaprov, an elderly Jewish couple, and young hispanic couple Rodriguez (Joseph Gonzalez) and his wife Maria (Marina Sirtis).
After a few violent muggings, Kersey decides to go into action: he uses money to buy a car from a used car shop as bait. When two gang members try to break into the car, Kersey protests, and they reply "What's it to you man?" Kersey says, "It's my car!" Whereupon they laugh and say, "now you're gonna die!" and advance towards Kersey. Kersey quickly pulls out a revolver and shoots them dead.
Kersey protects Maria twice from Fraker's gang, but is unable to save her a third time as she is raped (she later dies as a result of her injuries).
Kersey orders an automatic gun for protection, a .475 Wildey Magnum, and spends the afternoon handloading ammunition for it stating "Nothing's good enough for our friends". He then tests the gun later that night when he kills another gang member, The Giggler (Kirk Taylor), who steals his camera bag and tries to make a clean getaway. He immediately becomes applauded in the neighborhood as Shrieker and the police become involved in the violent acts. Kersey also throws a gang member off a roof (landing on the hood of a car) during a nightly chase sequence.
There is also a subplot involving a possible love interest in public defender Kathryn Davis (Deborah Raffin) in which we learn why Kersey is hesitant to get back in the dating game. Kersey later goes back to say good-bye to her, as she is going out of the city to visit her sister. Kersey offers to take her out for dinner on her final night. While waiting in Paul's car, Kathryn is knocked unconscious by Fraker and the car is pushed into oncoming traffic. The car slams into another and explodes, killing Kathryn.
The police arrive and Shriker places Kersey under protective custody because Kersey is now in too deep. Bennett takes matters into his hands, after his taxi shop is blown up, in an attempt to drive out Fraker's gang with one of Charley's machine guns, a MG-42. The gun gets jammed as the gang cripples Bennett. Shriker notifies Kersey and takes him to the hospital. Kersey escapes after Bennett notifies him of the one remaining gun. Kersey and Rodriguez spend the day collecting weapons.
At the climax, both Kersey and Shriker attack a group of gathering gang members. They mow down many of the criminals with a Browning M1919 A4 machine gun before running out of ammo. Kersey then uses the Magnum while Rodriguez uses a zip gun, while many neighbors begin fighting back as Fraker sends in more reinforcements.
Shriker decides to help Kersey and together they kill many criminals before Kersey runs out of cartridges for his Magnum. He goes back to the apartment to collect more ammo, with Fraker in pursuit. Fraker tracks Kersey down to the apartment and Shriker arrives just in time, resulting in Shriker getting wounded in the arm and Fraker shot (however, he wears a bulletproof vest).
Kersey obliterates Fraker once and for all with a mail-ordered 66mm LAW rocket launcher. They both hear sirens, and Shriker gives Kersey a head start. Kersey gives a look of appreciation and leaves as the remainder of Fraker's gang retreat.