Detective Jack Williams, who lost his left arm in the Tet Offensive, is shot dead in his apartment. His estranged friend, detective Mike Hammer (whose life Jack saved while losing his arm) is warned by police detective Pat Chambers to stay out of it but he nevertheless investigates the matter on his own. He speaks with Jack’s widow Myrna, who says that they were attending a sex therapy clinic operated by the glamorous Dr. Charlotte Bennett. Hammer visits the clinic and finds a Government Issue bugging device in the doctor’s office.
Hammer’s secretary Velda identifies Jack’s receipts for gasoline near Bear Mountain close to a summer camp run by Hammer’s old friend Joe Butler. Mike and Velda visit Joe, who tells them of a military project in Saigon involving the use of drugs to turn prisoners of war into friendly spies and how Captain Romero developed a technique for mind control. Two cars of CIA agents pursue the three on a car chase that ends when Hammer throws a Molotov cocktail at one car, causing it to drive off a cliff into the water, and blocks the road with his vehicle then shoots the second car, causing it to explode.
The FBI trace the gun that killed Jack to special effects artist Harry Lundee, who had reported the gun stolen. Hammer visits him on set, where Lundee is shot in the back by a projectile knife fired by an unknown assassin and, in his dying breath, confesses that he laundered the gun to mobster Charlie Kalecki, but Kalecki is reluctant to speak with Hammer about any ties to Romero.
The CIA, wishing to distance itself from Romero’s experiments, plants a series of clues in an attempt to lead Mike Hammer to Romero in order to have Hammer eliminate Romero for them. Chambers is instructed by the CIA to plant a photo of Romero in Jack’s apartment as bait for Hammer and Romero gives Dr. Bennett a fake file about Jack’s activities, which Hammer is upset to read during a visit to the Northridge Clinic. Hammer questions the sexual surrogate twins who worked with Jack, before observing Dr Bennett and her sex therapy team at work. While watching this session, Hammer hears the twins being attacked but is too late to prevent their deaths at the hands of a psychotic killer. In the wake of these extreme events, Hammer checks in on Dr Bennett at her practice and the two become lovers.
The twins’ killer, Charles Kendricks, has been brainwashed by Romero, who sends him to abduct Velda. Romero’s black ops squad capture Hammer, torture him and cover the badly-beaten Hammer with cheap liquor, intending to push him to his death in traffic. Hammer turns the tables on his captors, fights his way free and escapes. He races to Kendricks’ apartment and stops him from killing Velda, then pursues Kendricks through the Manhattan streets and shoots him dead. Convinced that Kendricks was a puppet, Hammer confronts Detective Chambers. Chambers, again being secretly instructed by the CIA, tells Hammer that Kalecki supplied the gun that killed Jack and owned the apartment building where Kendricks lived.
Hammer captures Kalecki and forces him to drive back to the Northridge Clinic, where Romero has now set up a sequence of fortifications and death-traps. Hammer jumps out of the car before Romero sets off a mine, instantly killing Kalecki. Hammer kills all of Romero’s goons commando-style then climbs over a wall and into the main building to confront Romero. After a brutal fight, Romero wrestles Hammer’s gun from him but Hammer has plugged the barrel, so when Romero fires the gun it explodes in his face. Romero dies before Hammer can get the answers he wants about Jack’s death. Searching Romero’s office, Hammer finds Romero’s black ops computer files.
Later, Hammer visits Dr Bennett at her home, bearing an expensively wrapped gift that turns out to be Jack’s prosthetic arm. Hammer confronts her with the information he’s uncovered: she was the intruder who murdered Jack Williams. Charlotte attempts to seduce Hammer and kill him with a hidden gun but he beats her to it while they embrace, shooting her in cold blood. With her dying breath, Dr. Bennett asks Hammer “How could you?” Using the famous closing line from Spillane’s original novel, Hammer responds “It was easy.”