When the elderly and senile matriarch, Edna, portrayed by Robyn Nevin, mysteriously disappears, her daughter Kay, played by Emily Mortimer, and granddaughter Sam, portrayed by Bella Heathcote, make their way to their secluded family home in search of her. Upon arriving, they find the house securely locked from within and notice a peculiar black mould-like substance creeping across an upstairs wall. The once-familiar space is now littered with Post-it notes containing reminders for Edna, indicating her derangement.
As night falls, Sam is approached by their young neighbor Jamie, played by Chris Bunton, who discloses that he hasn't visited Edna for some time, due to his father's warnings about returning to the house. Compounded by unsettling creaks emanating from the walls and the proliferation of black mould, both Kay and Sam are plagued by dread. Kay's uneasy sleep is interrupted by dreams featuring a grotesque, decaying corpse hidden in an old forest shack.
The following morning brings a moment of relief when Edna mysteriously appears, disoriented but muddy and barefoot, seemingly unaware of her own absence. After a doctor’s assessment, it is determined that Edna retains much of her mental clarity and physical stability, save for a large bruise on her chest resembling the unfurling mould. Kay expresses her intentions to transfer Edna to a retirement home due to her evident neglect, a plan that deeply unsettles Sam.
That night, Kay awakens to find Edna in a trance, sleepwalking towards the front door. The chilling words escape Edna’s lips: “It’s nothing.” After smuggling her back to bed, Edna remains convinced of an unseen menace lurking in the room and implores Kay to look beneath her bed, where Kay is momentarily distracted by something ominous breathing beneath.
The next day, a poignant moment between Sam and Edna evaporates when Edna accuses Sam of theft, abruptly snatching a ring from her finger—an item she had gifted to Sam, forgotten in her confusion. This unsettling scene unfolds while a sinister black figure lurks in the shadows. Concerned for her grandmother, Sam seeks answers from Jamie’s father Alex, portrayed by Jeremy Stanford. Alex recounts a haunting incident during one of Jamie’s visits when Edna accidentally locked him in a hiding spot, leaving him confined for hours, entirely forgotten.
Back at home, Kay follows Edna into the garden and is horrified to see her ravaging their family photo album, compulsively tearing out pages to consume them. When Kay tries to intervene, Edna bites her hand and rushes away, intent on burying the album in the soil, convinced it would be “safer there.” Stricken with fear over what horrors might haunt her, Edna reluctantly agrees to allow Kay to care for her.
As Sam delves deeper into the house’s hidden passageways, she becomes disoriented, screaming and pounding against looping walls. Meanwhile, Edna's condition continues to deteriorate; she gazes at Kay with suspicion, refusing sustenance and exhibiting alarming signs of incontinence. Concern deepens as Kay discovers Edna's urine has turned black and finds her scratching at what was once a bruise, now rotted flesh. A series of traumatic events culminate as Sam navigates the treacherous corridors, ultimately breaking free from the suffocating mould.
In a horrifying revelation, Kay discovers Edna using a knife to pick at her own decayed face. Forced to flee into the passageway as Edna’s madness escalates, Kay narrowly manages to fend off her mother’s twisted aggression. Edna, now grotesquely deformed, gestures weakly to a post-it note on the ground that reads “I AM LOVED,” and manages to smile through her agony.
In a moment of clarity, Kay understands that the malignant mould has receded from the walls and returns to her mother, assisting Edna in shedding the last remnants of her appearance until she reveals her final state: a decaying corpse, mirroring Kay’s distressing nightmare. In an intimate tableau, Kay and Sam lie beside Edna as she slips into eternal rest, the line between love and horror irrevocably blurred.
The narrative culminates as Sam notices a small black bruise at the base of Kay’s neck, a haunting reflection of Edna’s original mark—hinting that, in time, Kay may face a fate eerily reminiscent of her mother’s tragic descent into madness.
