Judy Sanders, Robin Frates, an employee of a genetic research facility named EMAX, contacts Lori Tanner, Ally Sheedy, a television personality and animal activist. She plans to meet after work to show Lori the atrocities and animal cruelty that go on in EMAX's laboratories. As she returns to work, an animal assailant attacks and kills her, before being sedated by EMAX owner Dr. Jarret, Lance Henriksen.
Later that night, Lori arrives at EMAX with her camerawoman Annie, Trula M. Marcus. They break into the laboratory, film the various animals that are being experimented on, and free a Tibetan Mastiff named Max before escaping with him. Jarret immediately goes to the police and reports that his dog has been stolen. Later that night, a mugger, Thomas Rosales Jr., steals Lori's purse. Max chases the thief, brutally kills him, and retrieves the purse.
Unaware that Max killed the mugger, Lori develops a bond with him, much to the disapproval of her boyfriend Perry, Fredric Lehne. He insists Max must stay in the backyard. However, Max can understand human conversations and becomes protective of Lori. Jarret is questioned by the police about Max and reveals that Max is a genetically altered dog, spliced with the DNA of various animals such as big cats, snakes, chameleons, and birds of prey, giving him enhanced strength, speed, and senses. Max is also prone to violent rages, and Jarret regularly gives him a drug to keep him relaxed, but he fears that the effects of the drug will wear off.
Max acts loyal, obedient, and lovable to Lori, but he wreaks havoc in the neighborhood when she is not around. Max scares a paperboy, devours a cat and a vulgar parakeet, chews through the brake lines in Perry's car, and kills a mailman. In addition, Max forces Heidi, a collie that belongs to Lori's young neighbor Rudy, J.D. Daniels to mate with him.
Noticing Perry's animosity towards their new dog, Lori decides to find Max a new home. She takes him to a junkyard and leaves him with the owner Ray, William Sanderson, who assures her that Max will be taken to a ranch in a few days. However, when Lori leaves, Ray chains Max to the wall and beats him with a shovel to end his barking. When Max pulls loose from his chain, Ray burns his face with a blow torch but is quickly overpowered and killed. Max, now scarred, makes his way back to Lori's house. The police, after discovering the dead mugger and having Jarret confirm that Max was the culprit, now intend to stop Max at any cost. By the time Max returns to Lori's house, Perry has replaced him with a new puppy named Spike.
Max, feeling betrayed, burns Perry's face with his acidic urine, and attacks Lori before the police arrive, forcing Max to flee. An ambulance takes Perry away, and the police demand Lori's help to catch Max. Later that night, Max returns and kills the officers watching the house. In an attempt to get him back, Jarret kidnaps Lori and Spike in hopes that Max will follow them to the EMAX building, which he does. She first discovers him in the laboratory. Max relinquishes his aggressive, homicidal nature and begins to kiss Lori's hand. Jarret shoots Max with a shotgun before being knocked onto a large electrical cage, which kills him. Lori pets Max's head as he dies.
Three months later, Rudy's collie has given birth to puppies, most of which look like their mother, with the exception of a black puppy that resembles Max.
