Daisy Rain arrives at her apartment in the Lusman Arms, a former luxury hotel undergoing renovations, and is brutally beaten to death with a hammer by a masked intruder. The building’s atmosphere feels unsettled from the start, a place where whispering noises echo through walls and every corner hints at a darker history.
In the next scene, new tenants Nell Barrows and Steven Barrows, a teacher and a medical intern, move in with a wary sense of anticipation. They are greeted by Byron, the building manager, who introduces them to the amenities and a handful of residents. As Steven works long hours, Nell is left alone much of the time, which makes her crave connection. She befriends Julia Cunningham, a neighbor down the hall, and also chats with Chas Rooker, an elderly man who reveals the building’s shadowy past and hints at its creator, Jack Lusman, who vanished mysteriously and whose workers died under strange circumstances.
Days pass and the Lusman Arms seem to come alive with menace. Strange noises resound through apartments and intercoms. Nell discovers a trinket containing human teeth hidden in a wall, and both Julia and another tenant vanish under suspicious circumstances. The killer—a ski-masked intruder—has been killing and hiding their bodies, leaving the residents in fear. While probing Julia’s disappearance, Nell encounters Chas, who offers cryptic warnings about the building’s true nature and slips her a note that reads, “> Look for her in Room 504.” Nell soon learns that there is no Room 504, and the floors themselves appear to lack the numbers that would end in 4.
Seeking answers, Nell visits the Los Angeles Preservation Society, where she learns that Jack Lusman was an occultist who mingled science and magic. The symbols that decorate the building are part of an ancient spell, and the blueprints reveal a townhouse hidden within the structure—explaining the missing rooms on the other floors. Nell returns home and discovers a roof hatch that leads into the hidden townhouse, where she uncovers a chamber dedicated to the Golden Age of Hollywood, torture rooms, and dozens of corpses. The killer finally reveals himself, removing his mask to become the embodiment of evil that the credits call “Coffin Baby.”
A teenage resident uncovers that the webcam he used to spy on Julia captured her death, prompting him to inform Steven. He, Nell, Byron, and the doorman trace a passage into Coffin Baby’s lair. Coffin Baby murders Byron and the doorman and pursues Nell and Steven. Nell theorizes that Coffin Baby needs death itself and the Lusman Arms to sustain his existence. Chas appears, trying to guide them to safety, and cryptically murmurs that Coffin Baby was born when he clawed his way out of his dead mother’s womb.
Coffin Baby bursts from his lair, fatally hurling Chas against a wall and seizing Nell. Steven arrives just in time to bludgeon the monster, and a shelf collapses onto Coffin Baby. Police arrive and take Steven to the hospital as Nell returns to her apartment. Debris is cleared from Coffin Baby, who has vanished from sight. The creature crashes through Nell’s window again, attempting to finish the job, but sees the runes Nell had drawn on her arms and becomes momentarily disoriented. Two officers burst in, shoot Coffin Baby from the building, and he is hurled out a window, hanging from a cord Nell had wrapped around his neck. The officers check on Nell and then look out toward the window, only to find that Coffin Baby has once more vanished into the shadows.
