Now living in Bulgaria, retired LAPD officer Brett Anderson remains haunted by the traumatic death of his wife Ana. To help him heal with a change of scenery, Brett's friend Alex gets him a job as a security guard at an old apartment building in Sofia.Building manager Martin Baker and his associate Goran give Brett a tour of the building and explain Brett's monitoring duties. Martin gives Brett a phone number and instructs him to call it if her ever observes anything out of the ordinary, specifically in regards to an ominous door in the basement.Brett moves into an apartment in the building. Brett also meets local coffee shop barista Zara and the two of them take a mutual interest in one another.When Brett sees a shadowy blur on one of the security cameras, he calls the phone number. Blind man Jacob Keaton comes to the building to cryptically consult with Brett. Jacob explains that he once had Brett's job, but became a consultant after losing his eyesight. Jacob briefly listens at the door with strange carvings before leaving.Brett calls Zara to ask about the building's history. Zara reveals that eight-year-old twin sisters were supposedly murdered there by their father, who claimed their mother was actually responsible before hanging himself while in police custody. Brett later has a nightmare involving the twins.In the morning, Brett finds Martin copying the security files to take back to his company. Martin reiterates Jacob's insistence that they should not investigate the door directly.Brett brings Zara to the building. Brett explains how he retired from the force after killing a teenage gang member who murdered his partner. Zara spends the night with Brett, who continues experiencing haunting nightmares.Martin, Goran, and Jacob return to review the security tapes with Brett. After seeing footprints on the footage, Martin and Goran agree to investigate behind the door with Brett. Upon confirming the footprints, Martin and Goran hurriedly leave. Jacob warns Brett to never enter the hangar alone.Zara returns. Brett is confused when she asks him about having ignored her calls over the previous few days. While having sex, Brett sees Zara turn into a vision of his wife with her throat slit. Brett tells Zara that something is off about the building and his employers are hiding the truth from him.Jacob returns. Brett demands to know why the building is empty and why Martin and Goran became so frightened after investigating the hangar. Jacob takes Brett and Zara up to the penthouse where he shows them an invalid old man kept alive by medical equipment.Jacob introduces the man as Zachariye, the 150-year-old architect of the building who was also an alchemist that prolonged his life though unnatural means. Jacob explains that Zachariye is one of the guardians of seven gates to the afterlife, also known as 'the nightworld.' Zachariye was responsible for creating the seals that protect the gate upon which he constructed the building. Due to his impending death, the dead now threaten to open the door from the other side and overrun the land of the living. Jacob adds that only one man, Isaac Pritchard, knows the location of the other six gates. Jacob warns that Brett must help him prevent the dead from opening the gate.Alex shows up unexpectedly, claiming that he hasn't been able to contact Brett for five days. Brett realizes that the building somehow manipulated him into missing time.Jacob, Brett, Zara, and Alex investigate the hangar. Brett asks Jacob about the murdered twins. Jacob explains that their father brought them there because he believed the building could bring the dead back to life.Living dead phantoms in the shadows pull Alex and Jacob away. Zara is wounded in a similar attack, but breaks free. Meanwhile, Brett encounters a vision of his dead wife Ana.Brett and Zara escape together as the dead begin overrunning the hangar. Brett realizes that Ana made it through the door with them. Ana tries convincing Brett that he needs her to reopen the door in order to free her soul. Recognizing Ana as an evil spirit, Zara intervenes. The two women viciously fight until Brett musters the willpower to shoot Ana with a handgun.Brett and Zara escape into the elevator as the dead break into the basement. Once outside, Zara dies in Brett's arms. Brett looks back at the building and sees the real Zara pounding on a window. Zombie Zara resurrects and prepares to shoot Brett when a man in a black suit suddenly kills her.Flanked by more black-suited men, Isaac Pritchard introduces himself to Brett. Pritchard advises Brett that everything will be fine and his men will escort Brett somewhere safe. Pritchard sees Jacob in one of the building's windows. Brett sees Ana, Zara, and Alex, as well as the twins with their father.