Nathaniel Leech develops a tissue-regenerating serum for billionaire Charles Deaver who suffers from burns covering his body. Since the serum is experimental, Deaver orders his henchmen to "damage" Leech and inject him with it. The henchmen lock Leech away in the tunnel system under the hospital. As Leech´s body regenerates, his hunger drives him to ingest sewer rats for protein.
Meanwhile, maintenance worker Vince Stoker (Sizemore) and his niece Samantha investigate the hospital´s tunnels and become trapped underground with Deaver´s henchmen and Leech – transformed into a hideous rat monster - and must escape before Leech´s ravenous hunger puts them all on the menu..