Charles Deaver, a wealthy magnate scarred by a devastating car accident, seeks a risky lifeline by backing the work of Nathaniel Leech, a researcher chasing a serum that can regenerate dead cells. The scientist explains the chilling catch: as the serum takes effect, the patient develops an overwhelming hunger that must be kept in check with a special protein formula to prevent dangerous side effects. Deaver, determined to survive at any cost, orders his ruthless henchmen to brutalize Leech. In the vicious conflict, Krendal shoots the doctor and injects him with the serum, locking him away in a tunnel system to test whether the regeneration is real. With no protein administered, Leech’s hunger spirals, first driving him to feast on a rat and then, horrorfully, a dog, while his body begins to mutate into a terrifying hybrid.
A separate group arrives on the same property—salvage workers led by Vince Stoker—hopeful to strip an abandoned hospital of usable equipment. Among them is Vince’s niece, Sam, joining the expedition for the first time. As they explore the derelict corridors and descend into the tunnel networks, Leech reaches full monstrous form and begins stalking the intruders. The encounter becomes deadly when he kills Callum right before Sam’s eyes, a brutal reminder of the danger lurking underground. The surviving crew fights to escape, pressing deeper into the storage areas while the mutated Leech closes in.
Back above ground, Charles Deaver dispatches Krendal and Wilkes to retrieve Leech, unaware that both agents have their own hidden agendas. Krendal and Wilkes seal every exit except one, effectively trapping Vince and his team inside. They confront the raging monster, manage to shoot him, but Leech regenerates with terrifying speed and eliminates Wilkes in a pulse-pounding moment of survival and shock. The two sides—Deaver’s men and the salvage crew—tend to a fragile alliance as they realize the danger of letting Leech roam free, and Krendal reveals a shockingly dangerous truth: she is a double agent working to secure Leech’s serum for the U.S. government to develop supersoldiers. The revelation intensifies the stakes and complicates loyalties in a claustrophobic, high-tension sequence.
In a brutal turn of vengeance, Leech ambushes Deaver in his limousine and decapitates him, sealing the turn of the tide with a brutal display of power. To stop Leech from escaping into the wider world, Vince and Krendal lure the creature back into the tunnel labyrinth while Otis and Sam fetch construction gear to mount a counterattack. The plan comes at a steep cost: Krendal injures her leg while setting the explosives and sacrifices herself so Vince can complete the mission. In a desperate, close-quarters battle, Vince, Sam, and Otis hack Leech apart with power tools, tearing into his arms, chest, and head. Yet time runs out on the explosives, and a catastrophic failure traps Otis underground, killing him as Vince and Sam cling to a slim chance of survival.
A week later, Vince awakens in a military hospital, his body still tethered to the battle’s wounds and a new fear. He learns that he and Sam have been contaminated by Leech’s serum through the dying scientist’s blood, binding them to the same ravenous hunger that nearly destroyed them. A doctor connects Vince to an intravenous drip of the protein formula, a grim reminder that the cure might be as perilous as the illness itself. As the sterile room hums with quiet tension, Vince’s voice trembles with the dawning reality of the serum’s insistent cravings, and the audience is left with the unsettling sense that the nightmare has only just begun.
