As millionaire Charles Deaver (Richard Fitzpatrick), whose appearance is grotesquely altered due to a horrific car accident, faces the specter of death, he resorts to a desperate measure to escape his fate. He pours his resources into the innovative research spearheaded by Dr. Nathaniel Leech (James Binkley), who is racing against time while his wife Miranda battles terminal leukemia. Dr. Leech introduces Deaver to his groundbreaking serum designed to regenerate dead cells but warns of a dire side effect: it will incite a voracious hunger in the recipient, which must be regulated with a specialized protein formula to prevent catastrophic outcomes. Alas, Deaver’s ruthless enforcers, including his right-hand woman Krendal (Wendy Anderson), viciously ambush Dr. Leech. In a brutal assault, Krendal shoots him repeatedly and injects him with the serum before locking him in a deserted tunnel system overnight to monitor the results.
Without the crucial protein supplement, Dr. Leech's hunger spirals out of control, compelling him to devour a rat and later a dog. This insatiable craving ignites a horrifying transformation, turning him into a monstrous hybrid creature.
Meanwhile, a band of salvage workers, led by Vince Stoker (Tom Sizemore), infiltrates the dilapidated hospital on the property to scavenge for valuable equipment. Among them is Stoker's niece Sam (Amber Cull), who joins the perilous excursion. As they navigate the crumbling edifice, they unwittingly stumble upon the same tunnel system that has been the site of Dr. Leech’s horrific change. As they trigger his monstrous evolution, Leech begins a rampage, leading to the gruesome demise of Callum (Joe Dinicol), with Sam desperately sounding the alarm to warn Vince and Otis (Martin Roach).
Simultaneously, Deaver sends Krendal and Wilkes (Simon Northwood) to retrieve Leech, entirely unaware that they have fallen into a trap of lies. This duplicitous pair seals off all exits from the tunnel system, trapping Stoker’s crew within. Their paths collide with Leech's horrific form as they come face to face with the beast, leading to the swift downfall of Wilkes.
Initially at odds, Krendal and Stoker’s group decide to unite against Leech's relentless aggression. In a moment of stark revelation, Krendal discloses her true motives: she is a double agent working on behalf of the U.S. government, aiming to procure Leech's serum for the development of supersoldiers.
Leech seeks vengeance against Deaver, culminating in a shocking decapitation within the confines of his limousine, abruptly ending the billionaire's grip on life. In a desperate bid to neutralize the behemoth Leech, Vince and Krendal orchestrate a high-risk heist, drawing the creature back into the labyrinth of tunnels. Meanwhile, Otis and Sam embark on a hazardous quest to retrieve essential construction gear, gearing up for a climactic battle against the beast. Tragedy strikes as Krendal suffers a gruesome injury while setting explosives, sacrificing her safety so Vince can fulfill the remainder of their perilous strategy. Despite ultimately dismembering Leech with their power tools, their triumph is fleeting as the detonations take Otis’s life, leaving only Vince and Sam to narrowly escape the impending disaster.
In the chilling aftermath of this harrowing nightmare, Vince awakens in a military hospital, his body marked by the traumatic events he endured. With a week gone by since their encounter with Leech, the grim reality of their ordeal begins to set in as they discover they have been infected by the beast’s toxic serum through its blood. As the doctor departs the room, leaving Vince tethered to an IV drip of the protein formula, his body convulses in agony, the serum's sinister grip awakening an unquenchable hunger that threatens to consume him entirely.
