In futuristic 2008 London, England, global warming has caused the Thames River to permanently flood parts of the city, and rats have overrun London. Drinking interminable cups of coffee and eating chocolate, rogue police detective Harley Stone drives his Jeep through the watery streets, as fellow detective Dick Durkin tails him. At Jay Jay’s “non-stop dancing” nightclub, Harley telephones officer Pat O’Donnell at headquarters to report his instinct that the killer is going to show up. Hearing a scream, he runs into the bathroom and finds a woman murdered, her heart ripped out, and the words “I’m back” written in blood on the mirror. As Harley follows a drop of blood into a back alley, Dick Durkin displays his badge and introduces himself to Harley. Losing composure, Harley is subdued by several officers before taken to jail. Later, Dick learns that a serial killer murdered Harley’s partner, Foster McLaine, three years earlier, and he was overcome with guilt because he was having an affair with Foster’s wife, Michelle. Released from lock-up, Harley warns Chief Thrasher that the killer is back. Since Dick Durkin is an Oxford University-trained expert on serial killers, Thrasher appoints him as Harley’s new partner. Dick asks how Harley knew the killer would strike at the nightclub, but Harley refuses to talk. When Harley receives a box containing the nightclub victim’s half-eaten heart, he returns to Jay Jay’s and lies on the bathroom floor to make psychic contact with her. Sometime later, at another murder scene where a man’s heart has been torn out, Harley and Dick find Foster McLaine’s pistol in a plastic bag, and the astrological sign of Scorpio painted in blood on the ceiling. Officer Paulsen gives Harley a plaster cast of the nightclub victim’s heart, and he believes he hears the killer’s heartbeat. Telling Dick that the murderer is not far away, Harley shows his new partner the imprint of strange teeth marks in the plaster and wonders if the killer is a human or a beast. Leaving Dick behind, Harley goes to Foster McLaine's crypt and finds Michelle, who explains she visits her husband every time she returns to London. Concerned by Harley’s haggard look, she accompanies him home for the night and is shocked by his ramshackle apartment. When Harley falls asleep, she finds a photograph of herself with his detective’s badge. Harley awakens from a nightmare in which he walks through sewer water with Foster. As Michelle sleeps, Harley and Dick go for breakfast and coffee. Believing that Harley and the killer are on the same “psychic wavelength,” Dick wonders why the murderer strikes at high tide during the new moon, since Scorpio is a “water sign.” Meanwhile, the killer breaks into Harley’s Jeep to steal his shotgun, goes upstairs to Harley’s apartment, and sees Michelle taking a shower. Harley and Dick get a police call about an intruder, and they hurry back to find Michelle unharmed. They rush to another apartment when they hear a scream. As Dick enters, he is blown through a window by a gunshot blast. Harley returns fire at a fleeting figure and finds a dead woman in a bathtub. He stumbles outside, hyperventilates, and is sedated by arriving medical personnel. When Harley awakens at the hospital, he is shocked to see Dick alive, who explains that his bulletproof vest saved him. At headquarters, Paulsen tells Harley that the laboratory found a rat virus on one of the hearts, and fingerprints belonging to Foster McLaine. Dick receives a lab report that the killer shares the DNA structure of his victims, rats, and even Harley’s DNA. Harley suspects the assailant will go to the morgue because he did not have time to take the heart of his last victim. There, he and Dick see a beast and open fire, but it escapes. Shaken by the large, non-human killer, Harley and Dick return to headquarters to arm themselves with automatic weapons. Guzzling coffee, Dick surmises that the killer is Satan, who not only gathers his victims’ DNA, but their souls. Returning to his apartment, Harley finds a heart in his refrigerator. Michelle obsessively scrubs herself, claiming a rat bit her shoulder. Harley realizes that the bite has bound her to the killer. He kisses her goodbye, and runs downstairs to find Dick handcuffed and bloody inside his Jeep, but seemingly unharmed. Freeing his partner, Harley returns to his apartment and finds Michelle firing a gun at the beast as it scurries away. When Dick becomes woozy, Harley discovers that the beast etched the symbol of a pentagram into the flesh of his chest. Dick reads the sign backwards and learns that the satanic beast is headed for Cannon Street to continue his killing spree. The detectives get help from a man called “The Rat Catcher,” who leads them to the abandoned Cannon Street Underground train station. There, they find Michelle hanging by her wrists from a street grating above, as her legs dangle in the flood water. Seeing the beast lurking in the water, Dick climbs atop an abandoned railway car and instructs Michelle to swing toward him. As he grabs her legs, the beast surfaces, and Harley shoots it, then tries to electrocute it using a broken electrical cable. However, the creature attacks, and Harley reaches into its chest and rips out the monster’s heart. It continues to pulsate in Harley’s hand until he blows it apart with his shotgun. As Harley, Dick, and Michelle leave, something stirs beneath the water.