A psychopathic killer, later referred to as “Scorpio”, shoots a woman while she swims in a rooftop pool. He leaves behind a blackmail letter demanding he be paid $100,000 or he will kill more people. The note is found by SFPD Inspector Harry Callahan. The mayor teams up with the police to track down the killer, although to stall for time, he agrees to Scorpio’s demand over Callahan’s objections.
During his lunch break Callahan foils a bank robbery. He shoots two robbers and holds a third at gunpoint with his Smith & Wesson Model 29 revolver, bluffing him to surrender with an ultimatum:
I know what you’re thinking: ‘Did he fire six shots or only five?’ Well, to tell you the truth, in all this excitement, I’ve kinda lost track myself. But being this is a .44 Magnum, the most powerful handgun in the world, and would blow your head clean off, you’ve got to ask yourself one question: ‘Do I feel lucky?’ Well, do you, punk?
Callahan is assigned a rookie partner, Chico Gonzalez, against his opposition to working with an inexperienced police officer. The killer, Scorpio, is spotted by a police helicopter near Saints Peter and Paul Church. He was staking out potential victims (in retaliation to the mayor’s coded message to him put in a newspaper). Callahan and Chico are unsuccessful in finding him, and Callahan is briefly mistaken for a Peeping Tom. After assisting in preventing a suicide, Callahan and Chico learn that Scorpio has murdered a 10-year old African American boy.
Based on his blackmail letter, the police think that Scorpio plans to kill a Catholic priest next. The police set up a stake-out where the killer was first spotted. Scorpio eventually arrives, kills a police officer in the shootout that follows, then escapes.
The next day, the police receive another letter from Scorpio. He claims he has kidnapped a teenager named Ann Mary Deacon. He threatens to kill her if he is not given a ransom of $200,000. Callahan is assigned to deliver the money. He uses a tracking device so Chico can keep secretly following him. Scorpio instructs Callahan via pay phones to run around the city. They meet at the Mount Davidson cross where Scorpio gives Callahan a beating, and admits he intends to kill him and let Mary die. Chico successfully intervenes to stop Scorpio from killing Callahan, but Chico is wounded in the shootout. Callahan uses a concealed knife to stab Scorpio in the leg. The killer flees without the ransom and seeks medical help.
Callahan learns of Scorpio’s hospital visit and a doctor reveals to him that Scorpio lives in a room at Kezar Stadium. Callahan finds him there and chases Scorpio, shooting him in the leg. Callahan tortures Scorpio into confessing where Mary is being held, but she is found already dead. The district attorney reprimands Callahan for his conduct, explaining that because Callahan obtained his evidence against Scorpio (namely a sniper rifle in Scorpio’s possession) illegally, all of it is inadmissible in court and Scorpio is to be released as a free man. Stunned and outraged, Callahan continues to shadow Scorpio on his own time. The killer pays a man $200 to beat him severely and then publicly announces himself as a victim of police brutality.
Scorpio steals a Walther P38 handgun from a store owner and hijacks a school bus. He contacts the police with another ransom demand that includes a flight out of the Santa Rosa airport. Callahan jumps onto the roof of the bus from an overpass. After Callahan forces Scorpio off the bus, the killer flees to a nearby quarry and holds a boy at gunpoint. Having shot Scorpio through the shoulder, Callahan aims his revolver and reprises his ultimatum about losing count of his shots. Unlike the earlier encounter, Callahan does have one remaining bullet. Scorpio reaches for his gun, but Callahan shoots and kills him. Callahan removes his police badge, throws it in the nearby water, and walks away.