Now residing in Bulgaria, former LAPD officer Brett Anderson struggles with the haunting memories of his wife Ana's tragic death. In a bid to help him find solace, his friend Alex offers him a job as a security guard at an old apartment complex in Sofia. As Brett settles into his new role, he is introduced to the building manager Martin Baker and his colleague Goran, who provide him with an unsettling tour of the premises. During this tour, Martin hands Brett a phone number to call if anything strange occurs, particularly regarding a mysterious door located in the basement.
Moving in, Brett soon crosses paths with Zara, a local coffee shop barista, sparking a mutual attraction between them. However, when Brett spots a shadowy figure on the security feed, he decides to call the mysterious number. This leads to the arrival of the blind consultant Jacob Keaton, who once held Brett's position but lost his sight. After an odd conversation, which involves Jacob listening at the eerie door adorned with peculiar carvings, he departs, leaving Brett even more intrigued.
Curiosity gets the better of Brett, and he questions Zara about the building's dark history. She recounts the tragic tale of twin sisters who were murdered by their father—who believed their mother was responsible—before he ended his own life while in police custody. That night, Brett is plagued by nightmares featuring the twins and unsettling memories of his past.
The next day, Brett finds Martin in the process of copying security files, and once again, Martin emphasizes Jacob's warning against investigating the door. Later, Brett invites Zara over, where he opens up about his career, including the incident that forced him to retire after killing a teenage gang member. They share a night together, but Brett continues to suffer from disturbing dreams.
Things take a darker turn when Martin, Goran, and Jacob return to review the security footage. Upon discovering mysterious footprints, Martin and Goran decide to check behind the ominous door, but quickly flee in fear after confirming what they find. Jacob ominously advises Brett never to venture into the hangar alone.
Zara returns but is perplexed when Brett admits he ignored her calls. Their intimacy takes a twisted turn when Brett envisions Zara transforming into Ana, her throat slit. Disturbed, Brett shares his suspicions about the building and the secrets his employers are concealing.
Jacob reappears, prompting Brett to demand answers about the empty building and the scare Martin and Goran experienced. Jacob reveals the existence of Zachariye, an ancient architect kept alive by medical means, who once sealed the gate to the afterlife in the very building Brett guards. He explains that Zachariye is one of the guardians tasked with protecting the gate, and with his life dwindling, he fears the dead may breach the barrier and flood into the realm of the living. Jacob warns Brett that they must act swiftly to prevent disaster.
Unexpectedly, Alex arrives, alarmed that he has not heard from Brett for several days. It dawns on Brett that time in the building has somehow disintegrated. When Brett, Jacob, Zara, and Alex head to the hangar, Brett learns about the sinister past of the murdered twins—Jacob reveals that the father believed the building had the means to resurrect the dead.
Suddenly, they are besieged by phantoms in the darkness. Alex and Jacob are pulled away, leaving Zara injured but able to escape. Amidst the chaos, Brett encounters the vision of Ana, prompting a tumultuous confrontation where Zara steps in to protect Brett from Ana, now revealed as a malevolent spirit. With resolve, Brett gathers his strength and shoots Ana to protect Zara.
They flee into the elevator, pursued by the undead. Tragically, Zara succumbs to her injuries in Brett's arms, leaving him heartbroken. As Brett looks back at the building, he sees a spectral vision of Zara desperately banging on a window, only to be resurrected as a zombie, poised to shoot him. Just as tension peaks, a man in black—Isaac Pritchard—intervenes and kills the resurrected Zara.
Finally, Brett comes face-to-face with Isaac Pritchard, who assures him that everything will be alright, and his men will whisk him away to safety. However, the unsettling connection with Jacob and the haunted remnants of his past linger ominously in his mind as flashbacks of Ana, Zara, and the twins surface, intertwining the lines between life and death.
