In San Francisco, while police inspector Harry Callahan, called "Dirty Harry" by his colleagues, is at the crime scene of a young woman shot to death in a rooftop swimming pool, he notices a nearby high-rise building that has an unobstructed view of the roof. Climbing to the top of the other building, Harry finds a shell casing for a 30-06 high-powered rifle, then sees a handwritten note attached to an aerial. The note, which is addressed "to the city of San Francisco" and signed "Scorpio," demands $100,000 or Scorpio will kill one person each day, starting with a Catholic priest or a black person. The mayor decides to pay the ransom, over the objections of Harry, his boss, Lt. Bressler, and the chief of police, and instructs them to do what Scorpio has demanded, respond with a personal ad placed in The San Francisco Chronicle . The mayor then instructs them to add that it will take some time to get the money, hoping to give the police more time to find Scorpio. After the ad appears, Scorpio goes onto a rooftop in the North Beach area looking for a victim. While he is setting his rifle sight on a gay black man, a police helicopter crew spots him and, using a loud speaker, orders him to desist, but Scorpio quickly packs up and escapes. That night, Harry and his temporary partner, Chico Gonzalez, a college graduate who majored in sociology and is the antithesis of Harry, are driving through the Tenderloin District when Chico sees a man carrying a tan suitcase that looks like one the police saw Scorpio carrying. They stop the car and separate, after which Harry follows the man to a nearby apartment and, spying at him and his girlfriend through their window, determines that he is not the killer. Some time later, when the body of a ten-year-old black boy is found in a vacant lot, with a spent 30-06 shell casing nearby, Harry knows that Scorpio has killed again. The police establish a command center to set a trap for Scorpio in North Beach. There the police arrange for a particular building's rooftop to remain unlocked, near St. Peter and Paul Church, assuming that Scorpio will try to carry through with his threat to kill a priest. That night, while Harry uses binoculars to survey the area, Scorpio goes onto the unlocked rooftop. Harry yells to Chico to turn on the lights when he gives the signal, illuminating Scorpio on the roof. Harry shoots at, but misses the killer, precipitating a gunfight between the two. By the time Harry and Chico reach the other building, Scorpio has disappeared, but not before killing a policeman. The next day, Bressler shows Harry a letter that Scorpio has just sent in which he encloses pictures of fourteen-year-old Ann Mary Deacon and a demand for the $100,000. The letter, which also encloses a lock of Ann Mary's hair and one of her molars, states that she has been buried in a secret location and only has enough oxygen to live until 3:00 a.m. Although Harry is convinced that the girl is already dead, Bressler asks him to carry the money, at the mayor's request. Bressler orders Harry to go alone, as instructed by Scorpio, and tells Chico to take the night off, but Harry and Chico secretly obtain sensitive microphones that enable them to keep in contact at a distance. That night, Harry waits at the Marina until Scorpio calls him on a pay telephone, then sends him to various locations throughout the city, with Chico secretly driving to each point after hearing the instructions through the listening device. The final destination is beneath the cross on top of Mt. Davidson. Scorpio, who is wearing a ski mask, tells Harry to drop the bag of money and stand with his nose against the cross, then starts to beat and kick him, saying he has changed his mind and will let the girl die. Hearing this from a short distance away, Chico starts to shoot until Harry tells him not to kill Scoprio. In the melee, Harry manages to take a switchblade knife taped to his leg and stab Scorpio in the leg, sending the squealing killer rolling down the hill. Chico is badly wounded and taken to the hospital, but Scorpio manages to escape. Later, despite his own beating, Harry goes to question a physician at a local hospital who reported a man receiving treatment for a suspicious knife wound. Under questioning, the doctor remembers that the wounded man sells programs at nearby Kezar Stadium, where he has a small room. Accompanied by fellow detective DeGeorgio, Harry climbs over the locked stadium fence then finds Scorpio's room, where he has hidden his 30-06 rifle. Harry then sees Scorpio in the distance and chases him out onto the field. When DeGeorgio turns on the stadium lights, Harry has a clear shot at Scorpio and wounds him in the leg. Lying on the field, Scorpio screams and demands a lawyer, as Harry slams his foot on Scorpio's leg. Although Harry elicits a confession from Scorpio for killing Ann Mary, whose body was found where Scorpio said he buried her, as the district attorney angrily tells him the next day, his blatant violation of Scorpio's rights mean that the seized rifle and confession will be inadmissible in court. Frustrated and angry, Harry is convinced that Scorpio will kill again because he likes it and decides to use his own time to follow him. Scorpio soon becomes unnerved by Harry's constant presence and, in retaliation, hires someone to beat him so that he can tell reporters that Harry did it. Meanwhile, Harry goes to visit the recuperating Chico and tells him that he wants him to be his permanent partner, but Chico sheepishly says that he is going to get a teaching job instead. As Harry leaves the hospital with Chico's wife, she says that it is her fault, not Chico's, but Harry, whose own wife was killed by a drunk driver, gently tells her not to blame herself because the violence of his life is not for her and Chico. Soon Scorpio robs and attacks a liquor store owner, then seizes a school bus carrying several grammar school children. He then forces the terrified bus driver, Marcella Platt, to phone the mayor so that Scorpio can reveal his new demands for $200,000 and an airplane. The mayor is told to have the money delivered just off the Sir Francis Drake Blvd. off-ramp on the highway north toward Santa Rosa. The mayor asks Harry to deliver the money, but Harry angrily refuses then leaves the mayor's office. A short time later, as the bus exits the highway and approaches a rock quarry, Scorpio sees Harry standing on a trestle bridge. Harry jumps onto the roof as the bus passes underneath, then Scorpio starts shooting into the roof, causing the bus to barrel to a stop. The terrified children and bus driver escape, while Scorpio enters the quarry, chased by Harry. Seeing a young boy fishing in a nearby pond, Scorpio exits the quarry and takes him hostage. When Harry follows, Scorpio puts his gun to the boy's head, threatening to shoot unless Harry drops his gun. Harry quickly shoots and disables Scorpio, enabling the boy to rush away. As Harry approaches, the badly wounded Scorpio eyes his gun, prompting Harry to tell him that in all the excitement he himself does not know if he fired five shots or six and asks Scorpio if he feels lucky. Laughing, Scorpio starts to reach for the gun but is mortally wounded by Harry's powerful .44 Magnum pistol. Harry now opens his wallet and removes his badge, flinging it far out into the water.