Jeff Sanders leads a double life of sorts: by day, he is a simple, unassuming construction worker, and by night, an expert Kenpo student and master of his craft. Jeff's background is revealed; after losing his mother as a teenager, he became an outcast and frequently lashed out at his family and society in an attempt to assuage his anger. His father, Captain Sanders, gained the idea from a mutual friend, Kim, to enroll Jeff in a Kenpō school to better manage his rage and feelings. However, he lost his temper with a football player who punched his younger brother, and knocked him out with a powerful kick. Displeased with this event, Jeff's father forced him to move out of their home. Jeff, now estranged from his family and living alone, continued with his courses in Kenpo and eventually adopted Kim as a mentor and father figure. Jeff decides to return to his old neighborhood to visit Kim. Inside his shop, Kim is having trouble with local Korean mafia families, due to his refusal to pay them off and use his antique store to peddle drugs. Jeff helps out Kim and beats up the henchmen who attacked his store. A mysterious hit-man named Tanaka appears and kills the lead henchman due to his failure to force Kim to comply by head-butting him. He later kills Kim and although Jeff tries to chase him down, Tanaka escapes. Jeff vows to avenge Kim's death and is determined to find out who ordered Kim's murder. He remembers a boy named Jimmy who lived with Kim, and tries to find him to ask if he knows about the murder. However, Jeff's estranged younger brother Adam, now a cop, is investigating the case, and warns Jeff against trying to take matters into his own hands. In his hunt, Jeff is approached by a mafia boss named Yung who claims to be Kim's friend and points him to a fellow mafia boss named Sam. However, upon breaking into Sam's residence and attempting to kill him, Jimmy appears and reveals that Sam was one of Kim's closest friends and was the one who took him in for protection. Jimmy also clarifies that Yung is the one responsible for Kim's death, and was merely attempting to use Jeff as a pawn to kill his rival boss Sam. Jeff now plans to kill Yung, but Jimmy warns him that Yung is always protected by his hit-man Tanaka. In order to eliminate Tanaka, Jeff asks Jimmy to falsely testify that he witnessed Tanaka murdering Kim. Jeff has plotted to have Adam arrest Tanaka so that Jeff can get Yung alone to kill him. Adam and the police eventually capture Tanaka after a long car chase, but to Jeff's dismay Yung was absent from the car. Tanaka is knocked out with a taser, but later manages to escape from the police, breaking out of the police car and injuring Adam — and the driver — in the process. Jimmy overhears that Yung plans to escape the country by boat, and tells Jeff about Yung's drug factory. Now in a bigger hurry, Jeff sets out to attack Yung's drug factory, using his martial arts skills and various weapons to defeat the guards and employees protecting Yung. He eventually subdues Yung, but is attacked by Tanaka. Although Tanaka gains the upper hand during their fight, Jeff manages to kill Tanaka by setting fire to a nearby gas tank. Despite initially wanting to kill Yung, Jeff decides to capture him alive and turns Yung in to his father, Captain Sanders. The film ends with Jeff entering the kenpo dojo to visit his former master and a former fellow student.
The Perfect Weapon debuted at the box office at number six with a three-day box-office take of $3.9 million and had a total domestic box office gross of $14,061,361.
