Deep in the woods, twin brothers Samuel and Nolan live with their Momma and their dog Koda. Momma has taught the kids that the Evil has taken over the world and civilization is over, that’s why they’re hiding in this house. There are runes carved on the wood to protect them, and everyday they pray together to keep the Evil at bay.
If the Evil manages to touch them, it could make them do bad things. The kids can’t see the Evil, only Momma does, but they believe every word. One morning, a skeleton in the forest suddenly moves and screams, but this is just a dream from Momma.
The nightmare startles her awake, so she leaves the bed and after saying a quick prayer she goes outside. As she touches the grass, she can hear something growling nearby. Suddenly her dead father appears next to the stairs and tries to touch her, but she concentrates on ignoring it and goes back inside.
In the morning, she wakes the kids up to start the day. Samuel does some push-ups to get ready while it’s shown that Nolan has a bell tied around his ankle. During breakfast, Nolan shares his food with Koda, ignoring his brother’s disapproval.
Afterward the family gets ready to go foraging. Momma ties a long rope around everyone and they bless it with a prayer, that way the rope will keep them safe from the Evil. They all carry whistles in case they need to call for help, and Momma reminds the boys of the main rules: the rope is their lifeline, and they must never let go.
The rope is tied to the foundation of the house, a job done by the kids’ grandfather to protect his wife. Koda goes ahead alone, knowing he must look for rabbits. The family splits to cover more ground.
Momma continues to see the Evil pretending to be her dead father, who keeps whispering “let them go”, but she ignores it. When she finds a picture of her parents on the ground, she picks it up and hangs it on a tree. Samuel finds a squirrel and brings it down with his slingshot.
He also picks up bugs and eats a whole frog alive. Nolan climbs a tree to check a bird’s nest, but unfortunately all the eggs have already hatched. When he comes down, he sees an egg on the ground and runs to grab it, however his rope ends and stops him.
Desperate for better food, Nolan starts undoing the rope knot, only to be interrupted by Samuel who reminds him the egg could be an Evil trap. The brothers start making their way back together while arguing over what happened. Nolan hears Samuel say “she loves me more”, but Samuel swears he didn’t say anything.
Still angry, Nolan steps on his brother’s rope, which causes the knot to come undone. Samuel trips and falls, rolling down the mound and breaking his ankle. Nolan blows his whistle while Samuel hears something moving in the bushes, making him scared.
Seeing Samuel can’t move, Nolan decides to remove his rope to reach his twin and help him walk. At that same time Momma hears the whistle and runs to look for her kids. The rope doesn’t let her go very far, so she removes it too.
Now she can see the Evil following her while taking the form of her dead mother. Before the creature can reach the kids, Momma gets to them first and helps them get to Nolan’s rope, tying it around the tree of them. The creature appears behind them but can’t touch them because of the rope.
Momma is still very nervous, but the kids just stare at her because they can’t see the Evil. Once the creature leaves, the family returns home where Koda is already waiting. Momma is paranoid and with a knife in hand, she makes the twins touch the blessed wood and repeat the prayer.
The kids do so and nothing happens, which means they haven’t been touched by the Evil. Afterward Momma scolds them for what they did with the ropes and takes care of Samuel’s ankle. Later while she’s sitting outside, she sees her mother again.
During dinner, Momma decides to share a story. One time the Evil came to her pretending to be a little girl that needed help because her foot was stuck in a hole. Her screaming lasted many days and Momma did her best to ignore it.
When it finally stopped, Momma checked on the girl and found a dead body instead. Momma is sure that had also been a trick to make her believe it was real, but she didn’t fall for it. She tells the kids this so they remember to never let go of the rope even if it looks like an emergency.
Later that night Momma puts the boys in the cellar. They do this regularly to clean their souls and learn how to deal with the darkness. Each boy spends an hour there and then must share what nightmares they had with the family.
Afterward the kids get ready for bed and Nolan makes another creepy drawing for his wall. He asks Samuel if he saw anything after the rope fell, but neither of them did, which Nolan finds suspicious. He also wonders if their mother would kill them if the Evil touched them like she did with their father and her parents.
Samuel doesn’t like where Nolan’s conclusions are going, claiming that their mother would never lie to them. Meanwhile Momma is outside when she suddenly hears a voice: it’s her dead husband. This is the first time the Evil takes this form, and he starts begging to see the kids again because he’s sure his wife has no plan for the future.
She manages to ignore him even when his voice becomes creepier, but on her way back inside, she does pause when he points out they’ll starve. The next day Momma and Nolan check on the plants in their greenhouse, but most of them are dead or sick. They recently had a very harsh winter that killed their plants and scared off the animals, so food is scarce.
Nolan starts asking Momma questions about the old world, wondering how she can be sure there aren’t any survivors. Momma offers very vague answers and says this house is their only world now. As days pass, the family keeps on consuming all their stored food and soon they start to run out.
It comes to a point where they must cut just one tomato to share. On nights with a new moon, Momma lets the kids play an old record and they all dance together. One evening, Momma is taking a bath when she notices Nolan standing nearby.
He doesn’t answer when she speaks, meaning he’s sleepwalking. Momma takes him back to his bed and ties the bell on his ankle again, wondering how it fell. The next day Momma goes to the forest alone with her crossbow, hoping to find an animal to hunt.
She hears a bird nearby but when she comes closer, she finds it eating Nolan’s rotten body. The boy turns around to see her and Momma hides as she reminds herself it isn’t real. When she looks again, the bird is alone.
She gets ready to shoot it, but the Nolan illusion appears again and startles her, causing her to miss the shot and scare the bird off. Momma screams in frustration and has no choice but to gather tree bark for the next meal. During dinner, Momma keeps seeing her husband outside the window, which reminds her of her lack of plan.
The following day the family works together to gather more bark and hopefully find sap. Nolan feels something moving behind him, but he doesn’t see anything when he turns. Suddenly something pulls at his rope and Nolan falls as he’s dragged through the forest.
Eventually he discovers the rope is attached to a well where a snake-like creature is doing the pulling. At that moment he wakes up and discovers he had been dreaming while sleepwalking. He’s found by Momma, who furiously takes him inside and makes him touch the blessed wood while saying the prayer.
Then she throws him in the cellar and ignores his pleas for a potty break, causing Nolan to wet his pants. Momma doesn’t let him out until he swears the darkness has gone away. Once Nolan is back in his room, he tells Samuel that they should leave the house.
He thinks they should be able to outrun the Evil by working together, and promises to come back for Momma after they find something to eat. Nolan thinks she doesn’t know what it’s good for them anymore, but Samuel refuses. In her room, Momma overhears parts of the conversation and has visions of herself going feral.
The next day Momma brings out an old box. She shows the kids her Polaroid camera and explains how it works while showing them pictures of the family. The camera only has one picture left, and she’s saving it for a special moment.
Then she makes a big announcement: if they want to survive, they have no choice but to eat Koda. He’s getting old anyway, and they can’t feed a dog if they can’t feed themselves. Nolan immediately protests and cries, arguing with his mother as he points out killing family is the real Evil.
To make it fair, Momma asks Samuel to vote, and he agrees to eat to Koda. A devastated Nolan runs outside and hugs Koda, intending to protect him. However Momma takes him from his arms and drags the dog into the greenhouse, where she cries as she thanks him for everything.
Momma gets the crossbow ready to shoot yet she hesitates. At that moment Nolan appears behind her saying it’s not real and cuts her rope. Then he locks the greenhouse door so she can see the truth.
Momma immediately freaks out and start breaking the windows at the same time Samuel comes out and jumps on his brother, starting a fight. As the boys roll on the grass, the Evil appears for Momma using her mother’s face. The creature scolds Momma for all the things she did as child and promises she’ll make her eat her babies.
Not being able to deal with this anymore, Momma grabs a shard of broken glass and self-deletes in front of her children. Samuel finally punches Nolan to win the fight and runs into the greenhouse to tie the rope to his mother. He starts praying to help her, but nothing happens.
Feeling guilty, Nolan ties his mother’s rope around his body and goes back to the house. He plays the old record to process his grief while Samuel spends the entire night clinging to his dead mother. Sometime later Nolan looks for Koda, but the dog is now scared of the family and stays in the forest.
Samuel is feeling sick because of the hunger and Nolan makes tea for him. It seems Samuel hasn’t left their mother’s room since the incident, and he claims he saw Nolan in the woods laughing at him for not saving Momma. Nolan tells him it must’ve been a dream, but he still agrees to touch the wood and say the prayer to calm Samuel down.
Determined to save his brother, Nolan grabs the rope and starts exploring the woods. Since he has all the rope for himself, it’s long enough for him to leave the forest and found an actual road. He’s amazed to see such hard ground and starts yelling for help, but nobody answers.
Later when Nolan returns home, he’s shocked to be approached by a man called Cole. He explains he had been hiking in the area when he heard Nolan’s call for help and that he has food in his bag. When he’s about to offer a ride on his truck, Samuel comes out and thinks Cole is the Evil, so he threatens him with the crossbow.
Cole swears he doesn’t mean any harm and walks away, but Samuel shoots him anyway. A bleeding Cole runs into the forest and Nolan follows him, ignoring Samuel when he says it’s a trick. Following the bloodstains, he finds Cole with his phone calling 911.
Nolan tries to grab the bag and Cole stops him, however at that moment he finally dies. Afterward Nolan takes the bag home and finds tons of food cans, so he eats until he’s full. He tries sharing with Samuel, but he still thinks this is a trap from the Evil and turns the food down.
In the middle of the night, the hunger is too much to handle anymore and Samuel gives up, so he eats some of Cole’s food. He looks at the other objects from the bag and in a wallet, he finds a picture of Cole with his daughter. There’s also a flashlight, which he takes outside to play with the light.
Suddenly he hears a noise and goes to the greenhouse to investigate, noticing something moving inside. He’s shocked to see his mother standing up and he rushes to open the door, but now she’s gone and he can smell the rotten body. Then Samuel hears a voice and finds the girl from the picture behind him.
She asks about her father and Samuel pretends he knows nothing, but the girl recognizes her dad’s flashlight. Samuel begins apologizing for his mistake and the girls becomes afraid of him, so she runs into the forest. He immediately goes after her, but since she’s running in circles, soon his rope gets tangled among the trees and he can’t advance anymore.
Seeing the girl as proof his brother was right, Samuel decides to take off the rope. Then Samuel keeps on searching but freezes when he sees that the girl is climbing a tree with extra arms, meaning she’s the Evil after all. By moving through the treetops, the girl appears behind Samuel with a twisted body and a long tongue that touches his hand.
The tongue breaks and climbs under Samuel’s sleeve, making its way to his face to enter his body through his mouth. In the house Nolan wakes up and notices his brother is missing. He goes outside and starts pulling at Samuel’s rope, but he finds the end empty.
Suddenly he hears Samuel calling for him and Nolan runs to investigate, only for Samuel to appear behind him and cut his rope. At that moment Cole’s truck turns on its lights and Nolan sees the Evil with Samuel’s face, so he immediately runs back to the house. Hearing noises upstairs, Nolan goes to Momma’s bedroom and finds Samuel sitting on her bed.
A strange black substance appears on the roof above Samuel, who doesn’t react to his brother’s questions. Nolan grabs his hand and touches the wood while he starts to pray. This causes Samuel to start roaring and take out a knife, which he uses to attack his twin.
Nolan quickly dodges the attacks and runs out of the room, closing the door behind him. Samuel moves the bed to jam the door and puts on the old record so he can listen to music while using the flames from the fireplace to start a fire in the room. Then Samuel goes outside to watch the house slowly burn down.
As the smoke starts taking over, Nolan wets a rag to cover his face and escapes through a window. He slips and almost falls off the roof, so instead he decides to go back in through a different window. Behind a curtain, he’s shocked to find Momma calling for him, but he refuses to believe she’s real.
Outside, Samuel uses the camera to take a picture of himself with the burning house. Nolan runs away from Momma and opens the cellar. When she comes close to the edge, he pushes her in and closes the door.
Then he hugs the creature and remembers all the times his mother protect them to fill the cellar with love as she used to say. The fake Momma body begins coming apart and reveals the creature’s real snake-like anatomy. It keeps struggling against in Nolan’s hold, but he never gives up and eventually his love causes the creature to blow up.
Sometime later, firemen and paramedics arrive to rescue the kids in a helicopter. As it flies away, Koda runs after it on the ground. Nolan notices they’re going to an actual town and tells Samuel that they’re free now, but Samuel only replies “she loves me more”.
Back in the house, the last picture shows a hand on Samuel’s shoulder.