Following a traffic accident that left him clinically dead for more than 80 minutes, a Southern California antique dealer named Hatch Harrison begins experiencing strange dreams and visions that connect him to a psychopathic killer, a young man who calls himself "Vassago". Vassago believes that he is the human incarnation of one of the demon princes of Hell, and that if he murders enough innocent human beings and offers them up in sacrifice to his Master, he will be allowed to return to the afterlife and rule at Satan's right hand. He also has a strange condition that enables him to see in the dark, but also causes his eyes to be extremely sensitive to light.
Meanwhile, the accident gives Hatch and his wife Lindsey, an artist, a new lease on life as they struggle to rebuild their marriage in the wake of their son's death from cancer five years before. As the couple set about trying to adopt a young girl named Regina, Hatch continues to be tormented by visions, in some cases even seeing through the monstrous killer's eyes. Making matters worse, Vassago slowly gains information about Hatch and his family in the same fashion, putting both Lindsay and Regina in danger.
It is eventually revealed that Vassago's real name is Jeremy Nyebern; as a teenager, he brutally murdered his mother and sister, then attempted to kill himself. His life was saved by the same doctor who miraculously resuscitated Hatch, Dr. Jonas Nyebern, Jeremy's father (thus facilitating the seemingly supernatural bond between the two characters). Like Hatch, Jeremy was clinically dead for more than 30 minutes, and during that time, believes that he went to hell and was later returned to do Satan's bidding. At the book's climax, Vassago's visions lead him to kidnap Regina and take her to his "hideaway" (an abandoned amusement park, where, as a boy, Jeremy committed his first murder). There, he is confronted by Hatch, who bludgeons Vassago to death with a crucifix attached to a flashlight, thus saving Regina and Lindsay. During the closing moments of this confrontation, Hatch inexplicably begins speaking in another voice and calls himself "Uriel" (whom Hatch later learns is an archangel mentioned in the Bible), thus implying that Vassago's beliefs about his demonic heritage and short-lived journey to the afterlife may not have been entirely delusional after all. Uriel/Hatch tells Vassago/Jeremy that instead of returning to hell as a prince, he will be returned as a slave.
After Vassago's defeat, the Harrison family strengthen their bond to each other, with Regina insists of calling her adoptive parents Mom and Dad.