The movie opens in Iraq in 2010. A small group of soldiers are running through a war zone in the desert when they encounter a man-made underground cave. The group leader, Lt. Griggs, sends two soldiers (Jimmy Tratner and Mike Santino) to investigate. Jimmy is recording. Underground, the camera light starts to flicker and then eventually dies out completely. Jimmy and Santino are heard screaming, and then the nightvision comes on to reveal hundreds of bats exiting the cave around the two men. Santino produces a light and walks over a river of human skulls to the back of the cave, despite Jimmy's pleas to leave. He illuminates the back wall of the cave, which has a Latin inscription on it.
The movie then flashes forward to 2013, in New York. Detective Ralph Sarchie is seen trying to give mouth to mouth to a very small bundle, which is revealed to be a dead baby. The coroner asks him repeatedly to let go, but he has trouble doing so and is clearly affected by the experience. The next scene shows Sarchie and his partner, Butler, patrolling the streets of New York. They hear a call coming in about a domestic dispute case and Sarchie says they'll take it. Butler, an adrenaline junkie, is excited because apparently Sarchie has a "radar" that causes him to take cases that take unusual, often violent turns. The call is about Jimmy Tratner. They pull up to his house and Jimmy answers the door. Sarchie needs to see his wife so Jimmy lets them inside. The wife, sitting on the sofa, lifts her head to show she has been badly beaten. Sarchie and Butler immediately try to arrest Jimmy, who fights back with a knife, slashing Sarchie's arm, and then sprints away. They eventually catch him, with Sarchie beating him so badly that Butler has to pull him away, and arrest him. Sarchie notices Jimmy's fingernails are bleeding.
After Sarchie gets stitches in his arm, the two get a call for the local zoo. Upon arriving, the police at the scene tell them a woman has thrown her 2 year old boy into the lion's pit. Luckily, the pit is being repainted, so the lions are not there, but the boy was badly injured. In the ensuing chaos, the woman escaped and the power went out. Sarchie and Butler split up and look for the woman. Sarchie notices the animals are going crazy and eventually finds the woman trying to dig into the ground with her bare fingernails. She is also constantly repeating the lyrics to a song by The Doors. While arresting her, they notice a man in a hood, presumably the painter, in the lion enclosure. Sarchie wants to talk to him but he just slowly walks away, farther into the lion's pit. Sarchie follows him when suddenly the two lions are out in the enclosure. He barely escapes.
Upon taking the woman, named Jane, to the station, they are met by a priest named Mendoza. He wants to know more about Jane's behavior, saying he's been close with her for years and that she isn't insane. He says there are two types of evil: secondary evil, which man does, and primary evil, which is something else entirely. He believes Jane is possessed. Sarchie doesn't believe the priest but takes his card anyway.
The two are filling out the paperwork for the night when Sarchie gets a video message from his wife, Jen, and his young daughter, Christina, saying goodnight. Butler tells him the zoo will get them the video for the night's incidents in a few days. Sarchie overhears two other detectives talking about a case where a woman got a call from her father, who's been dead for 7 years, to "shut the door, the damn door". Seeing a connection between the woman in the zoo and this case (the Doors), Sarchie offers to take the case off their hands. He and Butler go to a house where the family says they think the house is possessed and that strange sounds have been coming from the basement, which was recently being painted. None of the lights in their house work, none of the candles will burn, and every time they replace a lightbulb it burns out within a few hours. Sarchie and Butler go down the stairs to investigate. The place reeks and hasn't been touched for some time. Butler goes upstairs for air, and Sarchie's radio starts making static for no reason. He eventually finds a body in the basement rolled up in a tarp, later to be revealed to be Lt. Griggs. Sarchie questions the family and they identify his picture as a painter. They say there was another painter too, but they didn't see his face as he always had a hood on.
Sarchie goes home to see his wife and daughter. The next morning Jen and Christina go to church, but Sarchie doesn't go. Jen tells Christina he used to go to church but doesn't anymore. He doesn't believe anymore.
Sarchie and Butler go to Griggs's apartment and discover he is Jane's husband. His apartment is a wreck. They find another picture with Lt. Griggs, Jimmy, and a man named Santino in it. Upon looking him up, they discover all three were dishonorably discharged for attacking an army chaplain. They served time and got out a couple of years ago. Sarchie recognizes Santino as the painter from the lion enclosure and they further infer he is the painter from the house. Lt. Griggs was found to have killed himself by drinking paint thinner, but Santino had to have been the one to wrap him in the tarp.
The zoo has finally sent over the footage from the night the detectives were there. First, they watch the video of Jane. She is seen walking past the lion pit with a stroller when she spots Santino there, painting over something that looks like an inscription. He turns to face her and she starts to shake violently and then throws her child into the pit. He turns back and continues painting as if nothing happened. Next, they watch the video of Sarchie in the lion pit. Santino is shown walking away from Sarchie and turning a corner to let the lions out of where they're being kept, seemingly talking to them. Sarchie hears static again even though his radio is off and children's laughter. Then, a bloodied man pops up on the screen for a split second, startling him. Butler doesn't hear or see anything.
Meanwhile, there is a scene where Christina is home going to sleep. She hears scratching under her bed and runs to her mom's room screaming. Sarchie goes home for the night and is met by Jen, who is angry at him because he yelled at his daughter and because "even when he's home he's not really there." He gets mad and tells her he's had a terrible week. She says he doesn't talk to her anymore and he says he'll try to be better. He goes to check on his daughter and finds nothing in her room until he sees the bloodied man over her bed in the mirror. He whips back around and the man is gone.
Sarchie continues to hear static and children's voices, so he goes to meet the priest Mendoza at a pub. Mendoza is drinking but explains he used to be hooked on heroin, and that it eventually led him to God when he hit rock bottom. He lets Sarchie listen to an audio clip of an exorcism of a woman Claudia and her 5 year old daughter. The clip deeply affects Sarchie. Sarchie lets Mendoza take a look at the clips, and Mendoza doesn't hear the voices or see the man. However, Mendoza claims Sarchie's "radar" might actually be a heightened awareness to all things occult and evil, which is both a gift and a curse.
Throughout the movie, Sarchie's family situation is becoming worse. He is distant from his wife and Christina's room is seemingly becoming increasingly haunted. One night, she hears the scratching again and her door won't open. Another night, her toys come alive on their own and move toward her. Jen is mad at Sarchie, saying Christina doesn't feel safe in her own home because she needs her father.
Jane, in a mental ward, incapacitates the physician and escapes.
Sarchie goes back to Jimmy's wife's house and sees the recordings from the war, including the one from the beginning of the film. Mrs. Tratner says she hasn't seen Jimmy since he got arrested, though he is out of jail. Sarchie views the recording and sees the inscription. He discovers the inscription is everywhere Santino is seen painting over - the zoo, the house with the haunted basement, the cave in Iraq, even Jimmy's apartment. Mendoza explains the inscription is a portal to Hell. This explains why all the references are to the Doors, a type of portal.
Butler and Sarchie find Santino's address. They, along with Mendoza, go to his apartment to wait for him. Butler is covering the front and Sarchie is in the car with Mendoza in the back. Mendoza says Sarchie needs to confess to God for him to help any further. Before Sarchie can do so, he spots Santino coming back into the apartment complex. He and Butler enter his apartment but he isn't there. They try to chase him down but they get split up, resulting in Sarchie getting attacked by a possessed Jimmy, who appears in the basement, and Butler getting trapped on the stairs. Mendoza is able to subdue Jimmy with his cross. Butler is stabbed to death in the stairwell by Santino.
Back at Mendoza's apartment, Sarchie is upset about not being able to save Butler. Mendoza tells him to confess to him. He does, and explains he has killed a man out of revenge before. Mendoza absolves him of his sins.
When Sarchie is driving home, Jane jumps to her death onto his car. When he is calling it in, he gets a call from Jen's phone, only to realize it's Santino on the phone, who has kidnapped his wife and daughter. He races home to find Santino and calls backup. They arrest Santino and bring him in. Mendoza and Sarchie are forced to do an exorcism. It is long and taxing for both, with Sarchie at one point only being able to hear a song by the Doors. Eventually, they expel the demon, and Sarchie finds his wife and daughter unharmed. It is revealed that after the birth of his second daughter, Sarchie retired from the police force and works with Mendoza to this day.
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