John, [William Metzo], a dapper and enigmatic figure in New Orleans, arrives at a dim bar in search of the "evilest" prostitute he can take home. He is directed to Shirley Anderson, [Laura Misch Owens], who agrees to leave with him for two hundred dollars. At his house, he asks her to lie on a table. She undresses, and John re-enters the room in a robe and wearing a mysterious metal mask. He begins massaging her and then ties her to the table and eviscerates her, removes her heart, and offers it on an altar in an Aztec sacrifice to Coatl, the "queen of evil."
Sergeant Frank Hebert, [Curt Dawson], and his partner Sergeant Mayer, [Ronald Tanet], are assigned to Shirley's murder case after her body is found on train tracks in the city. When questioning other local prostitutes, Hebert meets Sherry, [Gwen Arment], and discovers from her that the man whom Shirley had left with the night she died wore an unusual gold ring. Frank soon becomes enamored of Sherry, a hooker with a heart of gold, and the two begin falling in love, which clouds Frank's focus on the investigation. Meanwhile, Frank's superiors urge him and Mayer to prevent the murders from being publicized in fear that it will hamper the impending Mardi Gras celebration.
Meanwhile, John continues to stalk local strip clubs and bars for further female victims. He performs the same ritual murder on another prostitute. At a different club, he attempts to court another dancer but is confronted in the street by her pimp, whom John brutally stabs to death before fleeing. At another club, John is propositioned by Catfish, an eccentric hippie who is also a pimp, who offers John one of his girls. In the dressing room of the club, Catfish arranges for 19-year-old dancer and prostitute Sissy, [Nancy Dancer], to have sex with John to repay a debt.
At his apartment, John wines and dines Sissy with a meal of Chinese food at her request. However, when John prepares to perform his ritual murder of Sissy, he hesitates, sensing her inherent purity and lack of corruption. He initially orders her to leave but, unaware of his plans, insists that they complete their transaction. John continues to bind Sissy and proceeds with the ritual, murdering her. Meanwhile, Sherry becomes enraged with Frank after learning he pocketed money found with Shirley's body and vows never to see him again.
Sherry goes out for drinks at a local disco, where she gets into a drunken fight with another woman. Sergeant Mayer witnesses the incident and escorts Sherry back to her apartment. She tells Mayer she plans on leaving New Orleans as the annual Mardi Gras celebration begins the following day. Meanwhile, John meets Joe, a bartender, and offers him a large sum of money for three prostitutes for Fat Tuesday, during which he plans to provide three women to Coatl at once. One of the three Joe offers him is Sherry. John inebriates the women with pisco and prepares for the murder ritual. Meanwhile, Frank and Mayer are alerted to John's apartment by the Chinese food vendor who delivered there.
Frank and Mayer rush to the apartment and botch the ritual, saving Sherry and the other two prostitutes. John flees via a fire escape and leads Frank and Mayer on a chase through the French Quarter, ending at the Governor Nicholls Street Wharf. There, John steals a police car and crashes it into the Mississippi River. When they pull the vehicle from the water, they find the ritual mask, but John is nowhere to be seen.
