A group of treasure hunters led by Vincent Prather explore a forest, encountering a cave containing the coffin of Lilith, mother of all vampires. Vincent takes out a box containing the four sections of her heart and inserts it in her body, reviving her. Lilith awakens and kills the other treasure hunters, but Vincent subdues her with the key from Demon Knight.
After fighting with his sister Katherine, Caleb Verdoux goes to a dive bar, where an odd man named Jenkins tells him of a brothel hidden in a funeral home. Caleb and his friend Reggie visit the address, where they are forced at gunpoint by mortician McCutcheon to climb into a coffin leading to said brothel, unaware the prostitutes are all vampires led by Lilith. A prostitute named Tallulah seduces Reggie, whom Lilith kills before vampirizing Caleb.
When the police fail to find Caleb, Katherine reluctantly hires cynical and sarcastic private investigator Rafe Guttman, who tracks Caleb's trail to the bar, where Caleb's friends direct him to the funeral home. After a tip from Jenkins, Rafe revisits the funeral home that night. Initially, it appears as just a funeral home. Rafe overhears Lilith interview a woman, Tamara, followed by Tamara's scream and a body hitting the floor. He finds Caleb's nose ring and escapes. The next day he takes it to Katherine and tells her he's going to stake out the funeral home that evening. Unbeknownst to Rafe, the funeral home conceals a corrupt organization run by Reverend J.C. Current.
Rafe is admitted into the brothel and approached by the now-vampirized Tamara, whom he tricks into letting him strap her to a torture rack so he can investigate further, finding Jenkins decapitated in a coffin. While fleeing, Rafe drops his wallet, allowing Lilith to find his address. Having tasted Rafe's blood, Lilith takes an interest and tries to seduce him. When Katherine arrives, Rafe follows and describes the brothel's activity to her. They alert the police, who dismiss Rafe as a fraud due to lack of evidence. Meanwhile, Vincent destroys the key, freeing Lilith.
As Katherine reviews footage where she confronted Lilith, she notices Lilith is not in the shot. Realizing that Rafe might be right, she calls him over. Caleb calls for help, asking them to meet him at the power plant. Upon arrival, they discover the vampirized Caleb and flee, but Rafe falls out a window, landing on the police chief's car. The vampires catch Katherine and return her to the brothel.
Awakening in a hospital, Rafe is nearly killed by Tamara, who is posing as a nurse, but exposes her to sunlight, killing her. Katherine awakens and begs Caleb to free her, who refuses, too ingrained as Lilith's servant and having fully embraced his newfound vampiric lifestyle. Lilith prepares to feed on Katherine while Caleb watches. Rafe loads up on Super Soakers filled with holy water and raids the brothel, killing Vincent and McCutcheon. He meets up with Current, who, having realized his error and attempting to rectify it, has brought stakes and mallets to kill the vampires. Current tells Rafe that Lilith's heart must be removed from her body and quartered, as before she was found. Rafe gives him a spare water gun and the two enter the brothel and spray the vampires, including Caleb and Tallulah, who burn and explode.
They find Lilith, who mortally wounds Current and flees after Rafe attacks her with an axe. Rafe finds Katherine, and they head to Current's church to reveal the existence of vampires with its media equipment. Lilith returns, handcuffs Rafe to a railing, and attacks Katherine. Rafe uses a nearby laser to hit Lilith in the heart, quartering it. However, as the pieces remain in her body, Lilith remains alive. As Lilith devolves into a hideous form and attacks Rafe, Katherine grabs a candle stand and stabs out Lilith's heart. After Lilith's body burns and collapses to the ground, the two burn her remains and lock away the box with the heart pieces.
Later, in his car, Rafe fondles Katherine, which she seems oddly more accepting of now, having been disgusted by him earlier. When he pulls back her skirt, he notices bite-marks on her thigh, indicating that Katherine had been vampirized by Lilith while hostage. She then flashes her fangs and bites Rafe in the neck.
