In the sewers of a bustling city, sinister and unusual occurrences disrupt the mundane rhythm of the underground. A baby alligator, once a pet, was thoughtlessly flushed down the toilet, and through a diet of discarded laboratory animals used for a growth hormone experiment, it mutates into a behemoth. Over the years, the alligator survives and thrives, growing to an enormous size and amassing an insatiable appetite. The once innocent creature becomes a monstrous predator lurking beneath the unsuspecting city.

Terrifying incidents begin to surface as pets and residents mysteriously disappear, suggesting a threat from the bowels of the sewer system. A hardened yet troubled police officer, with a history clouded by a tragic incident, is assigned to investigate the bizarre cases. Teaming up with a determined herpetologist who warns of the dangers an alligator of such size would pose, they embark on a perilous journey to uncover the truth. The pair soon find themselves on a collision course with the now-monstrous reptile, as the body count rises and the city's foundations are threatened by the beast's rampages. The officer must confront not only the looming physical danger but also his own fears and guilt from his past.