When Edna, the demented, elderly, and widowed matriarch of the family, goes missing, her daughter Kay and granddaughter Sam travel to their remote family home to find her. They discover the house locked from the inside, and a strange black mould-like substance on an upstairs wall. Much of the furniture has been adorned with Post-it notes of reminders for Edna.
That night, Sam is visited by their young Down Syndrome neighbour Jamie, who states that he had not been to visit Edna for a while after his father had told him not to go back to the house. The pair grow increasingly disturbed by a loud creaking from inside the walls and the appearance of more black mould throughout the house; Kay experiences nightmares of a withered rotting corpse in an old shack in the woodlands.
The next morning, Kay finds Edna has returned, barefoot and muddied, but unaware of her own disappearance. Following a visit from a doctor, Edna is found to be mostly of sound mind, and physically stable, with the exception of a large black bruise on her chest (resembling that of the mould). Kay tells Sam that she plans to move Edna into a retirement home, given her self-neglect, much to the contempt of Sam. Late that night, Kay awakes to find Edna sleepwalking toward the front door, whispering, “It’s nothing”. After being put back to bed, Edna becomes convinced there is something hiding in the room and urges Kay to check under the bed; Kay sees something breathing but is distracted before she can investigate further.
Sam and her grandmother share a tender moment until Edna grows cold and accuses Sam of stealing from her. She snatches the ring off of Sam’s finger (not remembering that she had gifted Sam this ring) as a sinister black figure watches from the shadows. Following another violent event involving Edna, Sam visits Jamie’s father Alex to ask why he stopped letting Jamie visit. Alex reveals that the last time Jamie visited, the pair played hide-and-seek; however, Edna mistakenly locked Jamie in his hiding space and left him there for hours, forgetting she had even seen him that day. Back at the house, Kay follows Edna into the garden, where she finds her tearing pages from the family photo album and frantically eating them; when Kay tries to intervene, Edna bites her and storms off, attempting to bury the album in the soil, claiming it will “be safer there”. Fearing something may come to get her, Edna breaks down and agrees to let Kay take care of her.
Sam discovers a hidden passageway to another part of the house. She enters but becomes lost as the corridor begins to loop. Disoriented and hysterical, she resorts to screaming and banging on the walls. Edna’s condition seems to have worsened again; she now glares at Kay mistrustfully, refusing to eat and wetting herself. Kay notices her mother’s urine is stained black. As Edna bathes, she scratches what was once her bruise: it is now rotting black flesh. The bathwater overflows and reacts with an electric heater, causing the house’s power to go out. Sam travels deeper into the corridors. The ceilings slope lower, forcing her to crawl. She eventually escapes by knocking holes through the moulding walls.
Kay finds Edna, much to her horror, picking at her rotting face with a knife. Kay and Sam flee into the passageway (hearing the now crazed Edna approaching). Edna, now disfigured and physically contorted, crawls toward them; she attempts to pin down Sam before being beaten down by Kay. Wheezing, the decaying Edna gestures toward a post-it note on the floor, labelled “I AM LOVED”, and smiles. Kay realises that the walls of the house are no longer rotting and returns to her corpse-like mother to carry her to bed. She helps Edna peel away the last remnants of her hair and flesh to reveal her final form, a withered rotting corpse (like the one in her nightmare). Kay and Sam then lay on the bed with Edna until she peacefully falls asleep and presumably dies.
The film ends with Sam noticing a very small black bruise on the nape of Kay’s neck, resembling that of Edna’s initial bruise, suggesting that in time Kay will suffer a similar fate to her mother.