Monsturd tells the story of an escaped prisoner who falls into a sewage treatment plant and mutates into a giant fecal monster. The monster terrorizes a small town, causing chaos and hilarity ensues as the police and the townspeople try to stop it.

Mad scientist Dr. Stern is turning students into super-smart cannibalistic zombies, and the local cops are too busy chasing a sex fiend to stop the flesh-eating ghouls. Fortunately, Stern's nemesis, FBI agent Susan Hannigan, is on the case. As the ravenous monsters overrun the town, Hannigan tries to convince Stern's faithful assistant to reveal how to foil the crazed doctor's evil scheme.

Convinced that a small independent film crew is actually a Satanic cult committing real-life atrocities, Officer John Dardin takes justice into his own hands-leading to a blood-soaked massacre where fiction and reality collide.