The film opens in a 2010 firefight in a palm grove in the Diyala Province of Iraq. Three Marines discover an underground cavern and start to scream as their helmet video feed goes black. In The Bronx in 2013, NYPD officer Ralph Sarchie (Eric Bana) stands over the corpse of an infant in a darkened alley. He and his partner Butler (Joel McHale) resume their nocturnal patrol for the 46th Precinct. A domestic disturbance call comes in over the radio. Sarchie probes the dispatcher for more information and finds out that the male at the address is a former Marine. He tells Butler that his “radar” is going off because maybe the former Marine still thinks he is fighting in the war. At the site of the complaint, Sarchie and Butler encounter the shirtless and heavily tattooed former Marine Jimmy Tratner (Chris Coy) who insists his wife is okay. When his wife lifts her head, the officers see that she has been badly beaten. They notice deep scratch marks on the floor and are wary of a dog as they make the arrest. Jimmy resists them ferociously, eventually drawing a knife on Butler and fleeing the house on foot. Sarchie catches up to Jimmy and makes the arrest; although, not without suffering a sizable wound to his forearm that will require stitches. The officers notice that Jimmy’s nails are cracked and bleeding, and they assume that he is mentally ill or high on drugs. Sarchie and Butler are called to the Bronx Zoo after a woman has thrown her toddler into the moat surrounding the lion enclosure. They find the woman in a lemur pen. She is furiously scraping at the ground, and after they apprehend her, she rapidly recites the lyrics to “Break On Through (To the Other Side)”. Sarchie notices a painter inside the lion enclosure. He enters the pen to interrogate the mysterious man, but he is attacked by the lions and barely escapes. When the deranged woman, Jane Crenna, is transferred from the precinct to a mental health facility, a Jesuit priest named Mendoza (Édgar Ramírez) arrives at the family’s request. He asks the officers several pointed questions about Jane’s behavior at the zoo. When another domestic disturbance call comes in, Sarchie notices the complaint makes reference to doors and decides that he and Butler will respond. At the house, the family of three have been staying in the living room after a series of strange disturbances. There is one area of the house where light bulbs instantly burn out and candles will not remain lit. The family explain that there were two painters working the basement, where most of the disturbances occurred. In the basement, Sarchie discovers the badly decomposing body of one of the painters, David Griggs. At Griggs’ apartment, which is overrun by cockroaches and refuse, they find business cards for Alphonsus Painting company as well as a picture of Griggs with Jane Crenna and the child that she threw at the zoo. In another picture, Griggs is pictured in his Marine uniform with Jimmy Tratner and a third Marine named Santino (Sean Harris). They realize Santino must have been the painter at the zoo. Mendoza visits Sarchie at the precinct and asks to view the surveillance tapes of Jane at the zoo. He believes that Jane is possessed by demons, and he explains that there is secondary evil created by humans and primary evil which comes from demons. Sarchie is skeptical, but when he reviews the surveillance footage with Butler, he hears strange noises and sees things that Butler does not. Sarchie returns to Jimmy Tratner’s house and finds a wall that was being painted in Jimmy’s home office. He scrapes away the paint to find a pictograph of an owl. At Sarchie’s home, his daughter lies in bed as a stuffed owl stares ominously at her from a shelf in her bedroom. She is awakened by strange noises and frightened. Back at Tratner’s home, Sarchie has scraped off the paint from the rest of the wall to find a bizarre mix of Latin and ancient pictographs. Sarchie finds some hard drives with footage from Tratner’s deployments and watches the full footage from the palm grove in Diyala. In the cave, the soldiers found a carving of the same message that is on the wall in Tratner’s home. Sarchie revisits the basement where he found Griggs’ body, scrapes off the paint from a wall, and finds the message again. He reviews the zoo surveillance footage and sees the same message was being painted over by Santino in the lion enclosure. With Mendoza, he visits Jane Crenna in the mental hospital and shows her the message from the cave. She savagely bites Sarchie’s already wounded forearm. Mendoza decodes the message as a kind of bridge between Christian and pagan theology which would theoretically allow demons a door to the human world. He explains that certain people are more susceptible to such messages than others. He suggests that the voices and images Sarchie is seeing could be a result of his intuitive “radar”, which means that he is also susceptible to the archaic message. Mendoza and Sarchie gradually share more of their personal histories with each other. Mendoza goes with Sarchie and Butler to an apartment building where they are attacked by Santino and Jimmy Tratner. Santino overwhelms and eventually kills Butler. At Sarchie’s home, his daughter is once again awoken during the night. Her stuffed owl rolls off the shelf and advances towards her bed. As she runs screaming from the room, she sees Santino in the hallway. Sarchie arrives home to find Santino in his living room. Santino warns that he has abducted Sarchie’s wife and daughter. Santino is brought to the precinct where Mendoza and Sarchie perform an exorcism on him. Sarchie’s wife and daughter are located in an Alphonsus Painting van at a storage facility. The film ends with the baptism of the Sarchies’ second child.