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, 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 a basement (also shooting one of them afterwards), 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 Dobermann, 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 three more members of the Ghetto Ghouls (one of which being the gang member he’d interrogated earlier), 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. 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 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.