During a firefight in Vietnam, U.S. soldiers John Eastland and his best friend, Michael Jefferson, are captured by the Viet Cong. They are tied to wooden stakes with several other men, and tortured for information. When Eastland refuses to answer, the VC commander decapitates the soldier beside him with a machete. Jefferson escapes moments later, kills the remaining VC soldiers, and unties Eastland. Eastland then kills the VC commander.
The film then shifts to New York, where Eastland and Jefferson work in a warehouse. One day, Eastland catches a group of thugs, called the Ghetto Ghouls, trying to steal beer. He is attacked, but Jefferson comes to his aid. They defeat them, but the gang return to cripple Jefferson; by gouging his spine with a meat hook. Eastland, after this incident, captures and interrogates one of the gang members with a flamethrower. He then attacks the gang's base of operations with his rifle, shooting one gang member and leaving two others tied up in the basement, which is full of hungry rats.
Eastland's vigilante justice doesn't end there. The warehouse where he works has been forced into paying protection money. Gino Pontivini, the mob boss behind the scheme, has even taxed the workers paychecks. Eastland kidnaps Pontivini, and chains him above an industrial meat grinder. Eastland then demands information to get to Pontivini's safe, which Pontivini reluctantly gives. Eastland barely survives an attack by Pontivini's Doberman, so upon returning, he lowers Pontivini into the grinder for lying about the dog. Jefferson and his family are given Pontivini's money; to help pay their bills.
Detective James Dalton begins investigating the attacks, while the press dub Eastland the "Exterminator". Meanwhile, Eastland kills the ring leader of a child prostitution ring, as well as a state senator from New Jersey who sexually abuses children. He also kills a group of muggers, after witnessing them rob an elderly woman.
Meanwhile, the CIA has heard of the Exterminator and reaches an odd conclusion. Based on the current administration's promise to cut down crime rates, they believe the Exterminator is either an opposition party's stunt, or a foreign power's ruse to humiliate the current administration; by exposing their inability to handle the city's crime problem. They monitor Dalton's investigation of the Exterminator. And Dalton, working from a bootprint found at Pontivini's home, discovers the Exterminator wears hunting boots manufactured by a mail order firm in Maine. Asking them for a list of clients in New York, and following the hunch that the Exterminator may be a Vietnam War veteran; since he killed the Ghetto Ghouls with an M16 assault rifle, Dalton has narrowed his suspects accordingly.
Eastland visits Jefferson in the hospital, and because he will never be able to walk again, Jefferson asks Eastland to kill him. Eastland does, but coincidentally, Dalton is visiting the hospital at the same time. When he learns about Jefferson's death, Dalton surmises that one of Jefferson's friends was the Exterminator, and learns that one of his suspects, Eastland, was Jefferson's closest friend.
Eastland is aware that Dalton is staking out his apartment, so he arranges a private meeting with him, where he hopes to explain why he became a vigilante. However, the CIA are aware of the rendezvous after bugging Eastland's phone. They ambush him at his meeting with Dalton, which results in Dalton being killed while helping Eastland escape. And although he is presumed dead, Eastland survives, but in some territories, he dies as opposed to surviving.
Samuel L. Jackson appears in an uncredited role as a movie extra.
Rotten Tomatoes reports a rating of 22 percent, indicating mostly negative reviews.
At an advance screening; six months before the film's release, Roger Ebert, of the Chicago Sun-Times, criticized The Exterminator for being a "sick example of the almost unbelievable descent" that American movies had taken "into gruesome savagery". He called the film a "direct rip-off" of Death Wish (1974). After researching the Death Wish franchise, author Paul Talbot concluded that Death Wish "inspired" a number of "sadistic vigilante-themed movies"; The Exterminator being one of them. The New York Times, on the other hand, praised the acting, but felt the film was let down by the "screenplay and direction" of "beginner" James Glickenhaus; and by "lighting, camera work and sound that would rate an 'F' at film school".
Contemporary critics have been more appreciative. Eoin Friel, of the Action Elite, awarded three and a half stars out of five. Even though he found the opening scene quite "shocking", the acting a bit "ropey", and the effects "dated", he admitted the film "grabs you from the start". Den of Geek's Phil Beresford gave a mixed review. He stated that while Ginty is "not the greatest or most charismatic" of actors, "his essential ordinariness really works within the confines" of the film.
Due to its graphic violence, The Exterminator has been a controversial subject since its release. Dr. Sharon Packer and Jody Pennington spoke of this controversy and the film's "extreme justice" in their book A History of Evil in Popular Culture. They claimed the violence was a contributing factor to the film's "success," as the "audience loved it."
A sequel, Exterminator 2, was released in 1984. Ginty and producer Mark Buntzman returned, with Buntzman serving as the film's director; as well as producer. The sequel was less successful. It grossed $3.7 million at the domestic box office, while its predecessor grossed $5 million in the same region.