NOTE: This spoiler was submitted by Evan B

The film opens on Christmas night. A tub overflows with water and begins flooding the house. The camera follows the water as it begins seeping downstairs to the main floor. There stands an elderly woman named Edna (Robyn Nevin), seemingly in a trance. In the corner of the room, an ominous figure stirs in the shadow.

One year later, Edna’s daughter Kay (Emily Mortimer) and granddaughter Sam (Bella Heathcote) drive to Edna’s house. Edna’s neighbor calls them to check in because Edna has not been seen for a few days. They arrive at the house, which is full of creaking and noises. The pair find that many doors have new locks on them and the house appears to be deteriorating. There is clutter and mold everywhere. Edna also has placed post-it notes throughout the house. Some notes contain mundane information (such as “set the alarm”) and others are more sinister (such as “do not follow it.”). It seems like Edna is suffering from dementia.

They call the police and a multi-day search ensues. We learn that Sam and Kay live in another city and have grown distant from Edna (although Sam seems to have more warm feelings towards Edna than Kay does). Sam has a creepy experience inside a walk-in closet when something appears to be breathing next to her but nothing else is seen in the closet. Sam also learns that Edna’s neighbor’s son (who has down syndrome) refuses to come inside the house.

That night, Kay dreams of an old cabin in the woods. Her mother sits therein. When Kay enters the cabin, she finds a deteriorated old man sitting alone on his bed and covered in filth. Black mold begins covering the windows of the cabin.

Kay awakens and finds Edna in the kitchen making tea. Edna doesn’t tell her family where she’s been (and possibly doesn’t remember). The doctor is called. Other than finding a large black mark on Edna’s chest, she seems to be in good physical health. The doctor recommends that Kay and Sam stay with Edna to monitor her condition. Kay struggles to balance her work life with her family obligations and decides to look for a nursing home for her mother. Edna gives Sam a ring that is a family heirloom. As Sam continues to clean the house, she finds a sketchbook from Kay’s dad. The cabin from Kay’s dream is sketched therein. Kay tells Sam the cabin was the first house on the property and that Kay’s great grandfather lived there. The great grandfather eventually died (ostensibly of dementia), abandoned by his family. Parts of the old house were incorporated into the house Edna now lives in.

That night, Kay hears Edna seemingly talking to herself in the living room. Kay brings her mother back to her bedroom. Edna asks Kay to check under the bed. Kay thinks she sees something breathing underneath it, but is distracted before she can examine further. They then go to sleep.

The next day, Kay drives to tour an assisted living home (and cries after the tour at the idea of consigning her mother to such a home). Sam spends the day with Edna and the two have fun. Sam offers to move into the house to care for Edna. As the day goes on, Edna shows increasing signs of dementia. She accuses Sam of stealing the ring she gave her and attacks Sam to get it back. Sam later finds Edna staring in the walk-in closet. Edna explains that she thinks “it” came in through the closet. She also tells Sam that the house seems unfamiliar and that memories are being lost within the house. At night, Sam is scared by the washing machine randomly turning on and off. Kay has the same dream of the old cabin. It is clear that the mold covering the window in Kay’s dream is the same color as the mark on Edna’s chest.

In the morning, Sam finds Edna talking to herself and cutting her hand with a knife. When Sam tries to stop her grandmother, Edna throws a violent tantrum. Spooked by her grandmother’s behavior, Sam visits Edna’s neighbor to ask why his son is not allowed over anymore. The neighbor explains that Edna played hide and seek with his son and locked him in the walk-in closet for hours. When the neighbor came over to check on his son, he heard him screaming and trying to claw his way out of the closet. Since then, the son has not been allowed back inside. Meanwhile, Kay follows Edna into the woods outside her home. She finds Edna trying to eat photographs and bury photo albums. Edna explains that she thinks it is the only way to keep these memories safe from the thing Kay sees in the house. Kay tearfully embraces her mother and asks Edna to move in with her. Edna agrees.

Sam returns to the house and goes back to the walk-in closet. Sam sees a hallway obscured behind a pile of stuff. Sam goes into the hallway and discovers that there is an entire labyrinth within. She tries to turn back but is already lost. The house even seems to be changing around her. Sam becomes increasingly distressed as she wanders through the labyrinth. She also finds post-it notes with her grandmother’s handwriting, suggesting that this is where Edna was lost.

Kay and Edna return home and try to find Sam to no avail. Kay draws a bath for her mother. When Edna gets in, we see that the black marknow covers her entire chest. The bathtub begins to overflow and Kay returns to check on her mom, only to find she is locked out. When Kay looks through the window to the bathroom, she finds Edna stabbing herself in the chest. Kay breaks down the door. Edna runs off and Kay chases her into the walk-in closet and then into the labyrinth.

When Kay catches up with Edna, she finds Edna peeling the skin off her face and legs. Underneath Edna’s skin is a black, demon-like dermis. What Kay thought was dementia appears to actually be a possession. It is implied that this same affliction is also what impacted Kay’s great-grandfather

Kay flees further into the labyrinth as the monstrous Edna pursues her. Kay eventually finds Sam within the labyrinth. Sam finds a steel pipe and begins knocking through the labyrinth wall and back into the house. Sam climbs through the hole as the demonic Edna approaches. Edna attacks Kay as she climbs through the hole, and then attacks Sam. Kay beats the demonic Edna with the pipe until she is mortally wounded. Kay and Sam begin to flee the house. As Kay departs, she hears “Edna” whisper her name. She remembers how her great-grandfather was abandoned in her dreams and also sees a post-it note from her mother about love. Kay stops running and decides to return to the disfigured thing on the floor.

Kay carries it up to Edna’s bedroom. Once there, Kay peels off Edna’s remaining human skin and hair, leaving only the near-dead creature in front of her. She kisses the pathetic creature and tucks it into bed. Kay then crawls into bed and cuddles it. Sam returns and also crawls into bed with her family. As they lay there, Sam sees a small black mark on Kay’s back that is just like the mark that was on Edna’s chest (suggesting Kay will eventually meet the same fate as her mother and great-grandmother). The film ends with a shot of the cabin window from Kay’s dream being enveloped by black mold.

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Although it's assumed Edna is suffering from dementia, she is actually being possessed (just like her grandfather before her). The possession causes Edna to lose her memory and identity, just like Alzheimers. Ultimately, Edna's daughter Kay decides to stay with her mother until her last moments when the dying demonic entity takes full possession of Edna's body. Kay's daughter Sam finds a demonic black mark on Kay's back, suggesting that Kay too will eventually be possessed.