Young Marisa Kendall purchases a live baby alligator, which she names Ramon. One day, her father, in a fit of anger, flushes the pet baby alligator down the toilet. Twelve years later, detective David Madison investigates a severed arm and a bloated dead Lhasa Apso dog found at the city sewage works. The dog’s owner, Mrs. Ross, recognizes the dog’s markings although the dog carcass is five times larger than her pet. Lou Gutshel, a pet shop owner, steals pets and sells them to Slade Research Laboratory. Gutshel tells Arthur Hill, a Slade researcher, he is scared because a cop came to his store asking a lot of questions. He wants to temporarily stop supplying animals, but Hill insists Gutshel continue because his research depends on it. Gutshel disposes of the dead test animals in the sewer and is attacked and killed by a giant alligator. David returns to the sewage works to examine Gutshel’s severed leg and then visits Slade. Hill assures him that they only buy animals from the city pound or breed them on the premises. If there is a shortage of animals, research is suspended and, after the dogs die, they are sent to the pound for cremation. When David asks if dogs ever escape and end up in the sewer, Hill says no. Later, when Hill alerts Mr. Slade of David’s visit, Hill is instructed to carry on his research and not talk to reporters. At a press conference, David shares little information about the killings and offers no motive to reporters. Soon, David and Officer Jim Kelly search the sewer where Jim just misses walking into the open mouth of a giant alligator. The men run away with the alligator in pursuit. Trapped in a tunnel with a stuck sewer cover, the alligator attacks Jim and drags him away. Later, David wakes up in a hospital bed where he was taken after panicking and yelling “Alligator!” up and down city streets. While David dresses, Police Chief Clark recommends that he stay at the hospital, but David is determined to find Jim. Soon, David and Clark consult with Dr. Marisa Kendall, a herpetologist, who tells David that alligators larger than fifteen feet don’t exist in nature. Also, the toxic conditions in the sewer would prevent an alligator from growing so large. David returns to the sewers with a squad of men. Meanwhile, Thomas Kemp, a reporter, searches the sewers to check out the alligator rumor. He snaps pictures of the creature as it attacks him. Later, Kemp’s camera floats into one of the sewer filtration tanks and several of his photos reveal a reptilian mouth with gigantic, razor-like teeth. Soon, a police SWAT team descends into the sewer. Marisa, a consultant to the operation, tells David that she had a pet alligator named Ramon when she was a child. On a city street, the giant alligator crashes through the sidewalk. As a group of stickball players run for safety, a patrol car swerves to avoid the kids and crashes. With his car door jammed, the officer climbs out the window. ƒThe reptile rips off the officer’s leg and heads down the street. Marisa tells David that the alligator will seek out water next. When troops surround the lake, the mayor tells the officers he has put Col. Brock, a big game hunter, in charge of the operation. David quits. At Marisa’s laboratory, David looks at a slide of the Lhasa Apso and discovers it was pumped full of growth hormone. He reasons that if the alligator in the sewer ate the test animals, it would explain its size. Soon, Marisa investigates Slade Laboratory where Hill reveals that although his company discovered how to make synthetic testosterone, they never marketed it because it gave animals an insatiable appetite. When David returns to the police station, Chief Clark asks for his resignation. David steals dynamite from the evidence room and tells Marisa he has been kicked off the force. Soon, Col. Brock recruits local hoods to find the alligator. When the alligator attacks Brock, the hoods run away when they hear his screams. Meanwhile, cops pursue the alligator in the canal, where several officers are killed or maimed when they fall in the water. During Hill’s wedding reception at a private home, the alligator attacks. In the mayhem, guests panic, the mayor is killed and the alligator destroys a limousine with Slade inside. David and Marisa follow the alligator’s path back to the sewer. David snaps on a gas mask and activates a timed dynamite bomb. When he tries to escape through a manhole cover, a car stopped on the street above the manhole prevents him. Marisa grabs the steering wheel from the driver and moves the car with just enough time for David to escape the series of explosions from the dynamite. Afterward, the alligator is blown to pieces. Later, another baby alligator, flushed down a drain, falls from a pipe into the sewer and examines its surroundings.