A young Cristina watches as her mother undergoes an exorcism. When her mother beckons Cristina to her bedside, the demon controlling her lunges at Cristina and leaves scratches on her arm.In the present, an adult Cristina waking up in a remote hut in Veracruz, Mexico. A man, Javi, asks her why she was in a region called La Boca. Luz, a Nahua bruja, looks into her eyes and proclaims, "She has it." Javi then forces Cristina to drink goat's milk to purify her soul. Cristina's cousin Miranda explains that she found her lying unconscious at La Boca and brought her to Luz and Javi to help her.It is revealed that Cristina is a Los Angeles journalist who returned to her hometown to investigate the La Boca ruins. She is addicted to heroin and continues to inject the drug with her remaining syringes while chained to her bed, insisting that she does not have a demon inside of her. Luz and Javi perform a ritual to exorcise the demon. Though Cristina remains skeptical, she accepts she is possessed after disturbing events occur: snakes enter her room, she finds a rooster inexplicably mangled in her cell, and sees candles burn out and lead to a dark corner in the room. During another ritual, Luz seems to pull animal teeth out of her abdomen, and unsuccessfully tries to extract a snake, which escapes back into her body.One night, she chokes Javi unconscious and tries to escape, but the salt line across the threshold of the door prevents her from leaving. She becomes a believer in the demon, Postehki, after she vomits hair and black liquid. Postehki is "the death god of broken things," and has inhabited her because she is chronically depressed from spending her youth in foster care and turned to heroin. The demon has called her back to her hometown so he can finish "consuming" her. She discards what's left of her heroin supply and becomes determined to defeat the demon.Cristina asks about a mysterious boy she sees occasionally. The group declares they don't know of any boy living nearby. Cristina discovers the "broken man ritual" from a book Miranda gave her. During another ritual, Luz temporarily subdues a convulsing Cristina. Postehki reveals itself to Luz, who seizes its heart, sacrificing her life to expel it and save Cristina. However, Postehki enters Miranda through the scratches Cristina gave her earlier. Cristina paints her face, taking over the bruja role from Luz, and they draw out Postehki. Cristina grabs its heart, throws it to Miranda, who stabs it - burning and apparently killing Postehki.Cristina's editor Carson is brought to the hut by the locals after he was found near La Boca. He's arrived to rescue her, but he's brought more heroin. Cristina is uninterested in the drugs, and instead determines that he has been possessed once she sees his eyes. She begins the ritual to save Carson, having fully assumed the role as the new bruja. (copied from Wikipedia)