In the fictional town of Ashton, a grim night ends with Ron Bell, Stan Schwartz, eaten alive by a massive swarm of carnivorous slugs right inside his living room. The next day, Mike Brady, Michael Garfield, a health inspector and sewer manager, along with Sheriff Reese, John Battaglia, head to Ron’s home to serve an eviction notice, only to discover a nearly skinless body and trails of slime winding from the basement that defy easy explanation.
Later, Mike and his co-worker Don Palmer, Philip MacHale, are summoned to clear a sewer blockage, where Don uncovers several animal carcasses wedged in the pipes, hinting that something unthinkable lurks beneath the town. In the meantime, Harold Morris, Juan Maján, a local gardener, unwittingly slips a slug into his glove; the creature bites his hand, and chaos unfolds as he knocks over chemical jars, causing a fire that triggers a gasoline tank explosion, killing both him and his wife Jean Morris, Lucía Prado, in a brutal, fiery accident.
Back at home, Mike’s wife Kim Brady, Kim Terry, shares the news of the spread of danger as they discover abnormally large, aggressive slugs in their own garden. Mike collects a few specimens in a jar and consults John Foley, Santiago Álvarez, the town’s science teacher, hoping for answers. Foley witnesses one of the slugs attack and devour the class hamster, underscoring the threat the slugs pose.
The menace escalates as the slugs attack and kill two teenagers, while Mike’s friend David Watson, Emilio Linder, unwittingly consumes a slug that had been concealed in a salad prepared by his wife. The following morning, Mike and Reese are tasked with investigating the teen deaths, and Mike stubbornly suggests the slugs as the culprits, though his theory is dismissed by officials who distrust his claim.
Don then discovers that the locations of the murders align with the town’s sewer network, reinforcing the idea that the slugs’ aggression may be tied to something dumped into the wastewater. A separate chain of events unfolds as David falls ill in a restaurant, bleeding from the nose and collapsing as blood flukes burst from his eyes, a grotesque sign of the infection spreading through the town.
Seeing the pattern, Mike pleads with the town mayor to cut off the downtown water supply, hoping to starve the creatures, but his warnings fall on deaf ears. In the school lab, Foley introduces lithium-based arsenic that could kill the slugs in the sewers, and Mike convinces Don to join a plan to deploy it. Foley prepares a large batch of the arsenic, while Mike and Don storm the sewer system to confront the breeding ground.
That night, Don falls into the underground lair and is eaten by the insatiable slugs, a grim reminder of the danger they face. Mike then collaborates with Foley to pump the arsenic into the sewer using a firetruck hose, triggering an explosive reaction that destroys the slug population beneath Ashton, even though the detonation causes significant damage to much of the town.
As emergency services arrive and Kim reunites with Mike, the final twist reveals that, despite the widespread destruction, a single slug has survived the explosion, leaving a silent reminder that the threat may not be entirely eradicated.
