In Los Angeles, California, Sister Marguerite reports to her cardinal that fifteen ritual murders by the “Pentagram Killer” suggest that Satan is at work in the city. Meanwhile, Los Angeles Police Department (LAPD) homicide detective Russell Logan receives an anonymous telephone call from psychic Tess Seaton, informing him that another murder will occur near the Sunset Boulevard entrance to Elysian Park. The caller makes Logan promise he will not seek the death penalty if he captures the perpetrator. Drawing lines among the dots on his murder map, Logan sees they form an upside down pentagram, a sign used by Satanists. When Logan sets up a seventy-two-hour stakeout of the Elysian Park area, serial killer Patrick Channing chloroforms Carmen, one of the undercover officers, and prepares to kill her in the center of an upside down pentagram drawn in the dirt. Hearing that Carmen is missing, Logan and his partner, Detective Oliver Franklin, rush into the park and rescue her. Despite being knifed in the stomach, Logan subdues Channing long enough for other policemen to arrest him. Logan is hospitalized, but heals quickly. When the prosecutor seeks the death penalty against Channing, Tess Seaton again telephones Logan to warn that Channing’s execution must be stopped, but Channing dies in the gas chamber. Logan wakes from a nightmare about the execution and hallucinates that a room in his apartment is smeared with blood. Police arrive to take him to Elysian Park, where Carmen has been murdered in the same place he earlier rescued her. Her throat has been cut and a pentagram drawn in blood on her stomach. Tess Seaton drives to police headquarters, and as she parks in the underground garage, she sees someone who appears to be Patrick Channing. Upstairs, Tess introduces herself to Logan as the anonymous telephone caller, and tells him that she is a professional psychic who often knew what Channing was thinking. Now that Channing is dead, his spirit is free to create even more mayhem than before. She gives Logan her card and leaves. Meanwhile, police arrest a catatonic junkie carrying the knife that killed Carmen. Logan’s interrogation of the junkie yields nothing, but as he leaves, he hears Channing’s voice say, “See ya around, buddy boy.” Convinced that Tess Seaton has set him up, Logan and his partner, Oliver Franklin, drive to her house above Los Angeles and break in when she fails to answer. Inside, Logan finds a file that Tess Seaton has assembled on him, complete with his horoscope. He checks her phone messages, and one is from Channing, who addresses Logan by name and tells him to be at a church near Olvera Street in downtown Los Angeles. When Tess arrives and accuses him of breaking in, Logan plays back the tape, but Channing’s voice is no longer there. Tess looks at a St. Christopher pendant around Oliver Franklin’s neck, sees a pentagram in its place, and warns him of danger. The detectives take Tess with them, and as they arrive downtown, she senses Channing’s presence. When a horse-drawn carriage runs through a crowd of tourists, the horse tramples Oliver Franklin, whose dying words are, “It was him.” Logan finds the carriage outside a nearby ten-story building and chases Channing upstairs to the top. Channing leaps off the roof and lands unharmed in the parking lot below. He is gone by the time Logan reaches the ground. Logan’s bosses, Commander Al Perkins and Lieutenant Grimes, do not believe Logan’s story, and inform him that Tess’s lawyer took her home after threatening a lawsuit for breaking and entering. At St. Vibiana’s Cathedral, Logan confesses to a priest that he is a lapsed Catholic who has encountered something beyond his religious understanding. The priest speaks in Channing’s voice, and Logan chases him to the nearby Baltimore Hotel. Meanwhile, Tess has a psychic vision of Channing killing Logan with an axe. She races to the hotel in a taxi and reaches Logan in time to warm him, enabling him to dodge Channing’s axe. The killer rips down a ceiling fan and chases them with the rotor blades spinning. Logan and Tess escape and commandeer a car, but Channing leaps from a window and lands on the hood. Logan careens through alleys, banging into walls, until Channing is thrown onto the pavement. Later, Tess tells Logan that only one woman, Sister Marguerite, can help them. When they drive to her convent, the nun refuses to help, but warns that Channing has “the first power.” Logan and Tess stop for a drink at a bar. She senses Logan’s disillusionment since the death of his father, and tells him he needs to regain his faith. They drive to the house of Channing’s grandmother and pass themselves off as reporters writing a humanizing article about her grandson. The old woman takes them into the basement, where she maintains Patrick Channing’s boyhood room as it was. She tells them that Patrick never knew his father, and his mother died young. Feeling “vibrations,” Tess relives the night Patrick Channing’s grandfather came into the room and raped the boy’s young mother. When Logan accuses the grandmother of knowing what happened, she orders them out of the house. Still in a trance, Tess runs into a nearby water tunnel next to a reservoir. Logan finds her in a chamber inside the tunnel, which she recognizes as Patrick Channing’s “safe place.” Meanwhile, at the convent, a priest catches Sister Marguerite stealing a box containing a silver dagger with a crucifix handle. With resolve, she tells the priest she must stop a disciple of Satan who has been granted “the first power,” and the priest promises to pray for her. As Logan and Tess drive, his police radio reports an officer “needs assistance” at a nearby location. Logan takes the call, arrives at a warehouse, and goes in. Sitting in the car, Tess hears the radio repeat the message, but in Channing’s voice. The voice assets that Logan is the one needing assistance. She runs into the warehouse and arrives just as Logan falls through a rotted walkway to a floor below, leaving her alone with Channing. The killer chases her up some outdoor stairs, but Logan catches up and tackles him. When Channing appears ready to kill the detective, Tess hits him with a metal bar, knocking Channing off the platform. He is impaled on steel rods below, but when Logan and Tess make sure he is dead, the body belongs to Logan’s superior, Lt. Grimes. Later, as Logan and Tess embrace and kiss in his apartment, a bag lady who was sleeping outside the building floats up and crashes through a window. Logan grabs Tess and they run to his car. As they pull away, the bag lady pops up in the back seat and grabs his neck, causing him to crash into another car. When Logan regains his senses, the bag lady and Tess are gone. He returns to the convent and pleads with Sister Marguerite for help. The nun explains that there are three powers bestowed by either God or Satan: the first is resurrection and immortality, the second is the ability to know the future, and the third is the power to take over another body. Tess, she says, has the second power, but Channing has the other two. Guessing that Channing will take Tess to a special “dark place” where he performs rituals, Sister Marguerite displays the silver crucifix dagger, the only thing that can stop him. They hurry to the tunnel and find Tess lying inside a pentagram in Channing’s secret chamber, but the bag lady attacks them. After the crazed woman stabs Sister Marguerite, Logan shoots her in the head until she drops. The wounded nun tells him to remove Tess from the pentagram and replace her with the bag lady. Using a candle, Sister Marguerite sets the pentagram on fire, and flames engulf the bag lady’s body. Logan, Tess, and Sister Marguerite escape by crawling through a small tunnel, but when the nun appears to faint, Logan goes back to get her. Suddenly, she grabs Logan’s hand, speaks in Channing’s voice, and drags him back to the chamber. As she points Logan’s gun at his head, he appeals to Sister Marguerite to fight against Channing’s possession. To distract the evil spirit, the detective reveals that he knows Channing’s grandfather is also his father, and that the grandfather was probably the first person he killed. Recovering from Channing’s control, Sister Marguerite tries to stab herself with the silver dagger. However, she disappears, replaced by Channing, who attacks Logan. As they fight, Tess turns a wheel that floods the tunnel and carries them away in the torrent. Landing on a steel grate above a boiling vat of acid, Logan knocks Channing into the vat, flicks his cigarette lighter, and drops it. The boiling acid becomes an inferno. Logan and Tess run past the vat, but a flaming, disfigured Channing leaps out and grabs them. In the ensuing struggle, Channing turns into Sister Marguerite, but Logan plunges the dagger into her heart. As police arrive in time to see Logan stab the nun, they shoot him. Moments later, the nun’s body turns into Channing’s. Later, in the hospital, as Logan is in a coma, Tess sits at his bedside and begs him to return to her. Logan grabs her around the throat, but she awakens to see him lying quietly. Then she hears Channing’s voice.