Just before dawn on the outskirts of Seattle, a man dons sunglasses and fits a silencer to a pistol. He then drives into the city and to a street corner where a policeman sees his car and approaches, but is promptly shot dead by the man. He then drives to a police impound lot where another officer is parking a car and the killer shoots him dead. The killer then parks at the back of a car lot and walks to a nearby diner. He goes to the bathroom to wash his hands, as his police badge briefly flashes from his gun belt, indicating he is a dirty cop. The cafe owner greets him, for he is Stan Boyle, a detective sergeant. While the proprietor goes to the kitchen to check on food cooking, Boyle sees a car pull up outside. he goes out to the car and gives the driver a carrying case with the weapon inside, but as he returns to his own car, the driver shoots him down with a shotgun and escapes down an alley. Boyle's partner is Lon McQ, a lieutenant and, after being awakened by his telephone on his dockside houseboat, learns from police Sergeant J.C. Davis of the shootings. McQ deduces the shootings to be the work of Manuel Santiago, a respectable businessman and shipyard owner who fronts for massive shipments of narcotics from Canada. McQ's suspicion about Santiago is hardened when he is fired on near his car by a hit-man, who McQ shoots dead.He then visits the hospital, where Boyle is in critical condition and comforts his wife Lois. McQ asks her about the previous night where they were attending a show and Stan got a phone call to leave. After McQ's questioning he drives to headquarters, where police captain Ed Kosterman, has rounded up street radicals he believes may be responsible for the shootings. Lon also greets city councilman Franklin Toms, a friend of McQ and a close one to Boyle and Lois. McQ greets Kosterman in the station commissary and he orders McQ not to harass Santiago, pointedly reminding him of a previous case where McQ brutally assaulted a hood and the police had to deal with backlash from several lawyers. He blows Kosterman off and drives to the shipyard, where he photographs Santiago greeting a suspicious type just before he drives to a downtown restaurant pub. It is there that a local news broadcast reveals that Boyle died of his shotgun injuries. When Santiago goes to the restroom and uses a payphone to inform someone of Boyle's death, McQ jumps him, rams him head-first into the mirror, punches him unconscious, and dumps him in a trough.Because of numerous complaining phone calls to the station, McQ s assigned to desk work until further notice by police brass, but instead of taking the punishment, McQ angrily quits the force, despite Toms coaxing him not to, though he gives J.C. the roll of film on Santiago and tells him to get an identification on the suspect with him. Several days later, following Boyle's funeral, Lois comes by McQ's houseboat where he comforts her and she suggests they should go away together to Canada on a vacation. Later, McQ shows up at the office of Pinky Farrell, a private detective, and asks to join him as he wants to investigate Boyle's murder and Santiago's supposed role in it and not be held up by the police umbrella. Though it could get him into trouble with the authorities, Pinky agrees to register McQ and, at police headquarters, J.C. gives him the photos he took, identifying the man with Santiago as Freddie LaSalle, a hired hit-man from St. Louis. Later, McQ later borrows $5,000 from his ex-wife Elaine and has a bittersweet face-to-face with his teenage daughter Ginger.That night, at a Seattle Supersonics game, McQ confronts Rosey, an informant/pimp who wants nothing to do with McQ. He shows Rosey a newspaper with some of the $5,000 and Rosey tells him that Santiago has brought in three nationwide hit-men for a drug heist. Later, McQ greets cocktail waitress Myra, who was an informant of Boyle and may have been closer to him that just that. She agrees to tell him that she knows about Santiago's hit team and that they intend to steal from the police. The next morning, McQ follows a state police detail van transferring several million dollars worth of powder to an incineration plant at a local hospital, but Santiago's hit team, disguised as laundrymen, jumps them, steal the powder, and shoot at McQ as he pursues them to their laundry truck. Lon chases them around the city and finally corners the truck, but turns out to be a different one, as well as its driver. At the police station, Kosterman is doubly furious at McQ for the shootout, his inability to inform the police beforehand, as well as Toms, who defends McQ and Kosterman also revokes McQ's gun permit.McQ for his part, purchases another gun anyway, then is told by his friend, the gun shop owner, about a new sub-machine weapon invented by the U.S. Army, the Ingram, a 9mm weapon that fires 20 rounds per second. McQ is so impressed with the Ingram, after test firing it on a trashcan of water, he borrows the weapon and half a dozen magazines of ammo, even though the Ingram is not yet a licensed weapon. He then drives to Lois' house for a drink, where Toms also stops, for a visit, and warns McQ to lay low because he is already on thin ice with Kosterman and the police department. He then receives a phone call from J.C. that the search turned up nothing. McQ though knows better and that Santiago has driven the junk around before hiding it in his office where the police already searched. Yet it blows up in McQ's face when he finds the powder stashed in Santiago's office, cornered by his henchmen, and learns from a furious Santiago that the powder he stole is, in fact, powdered sugar, as the drugs were switched with by certain officials, and now McQ realizes Santiago never was involved in the killing of Boyle or the other two officers. Santiago, with nothing to fear, lets McQ go, but decides to beat him up as revenge.McQ now suspects Kosterman may be involved in the killings, but gets more shocking news when he confronts Rosey at a nightclub and reveals that Boyle was a dirty cop. McQ initially does not believe it, but does so when he discovers Myra was shot to death by one of Santiago's men, who thinks she knows something. Soon after, McQ himself is attacked in an alley, where two semi-trucks crush his car to oblivion. He is taken to a hospital for X-rays, where J.C. tells him they took his car to the impound lot. Lon now realizes something about his car and he sneaks into the yard to retrieve the Ingram and finds traces of cocaine, the same kind that was switched with sugar earlier. McQ now realizes his partner was indeed dirty, but when he notices J.C., Kosterman, and a few other detectives showing up, he is suspected of the drug deal and hot-wires a police car to escape. The next morning, he goes to Lois' house, who has her bags packed to visit her folks out of town. McQ drives with her out of the city, reveals her husband was dirty, and suspects that Lois may have the cocaine hidden in one of the suitcases. He then notices a car tailing them and tells Lois to turn off onto a dirt road leading to the beach. The man gets out and starts shooting at their car, but McQ shoots back and kills him. They notice the gunman was actually Toms, who, too, was in cohorts with Boyle.As they retreat back to their car, two more show up, with Santiago in one of them. McQ leads the pursuing vehicles on a chase down the beach. When the gunmen start to shoot up Lois' car, McQ quickly pulls out the Ingram and opens fire, causing their car to roll over several times. McQ and Santiago's cars pull over and he orders McQ to give him the suitcase of cocaine. He throws it near Santiago's car and tells his bodyguard to get it. However, one of the surviving henchmen, in the damaged car, tries to take McQ out with a shotgun, but is killed, as well as the bodyguard, and Santiago. McQ then walks over and opens the suitcase and the cocaine is dissolved by the ocean water. As the local police and ambulances arrive, Lois is forced to turn herself in and drove off by officers. Kosterman and J.C. are also at the scene, who apologize for suspecting McQ all along. Thinking there is only one way to bury the hatchet, McQ leads them to a nearby bar for a drink.