The film begins with a young boy with really long hair named Jack (Jacob Tremblay) telling us how he came into the possession of his mother, who we only know as Ma (Brie Larson) via TV people in outer space and him falling from Heaven. He wakes up and says hi to all the things in the tiny ramshackle room he shares with his mother: the chair, the cupboard, the inkblot on the carpet. Jack is excited because today is his fifth birthday. His mom tells him they’re going to make him a birthday cake. They go throughout their normal day she leads him through yoga practice in their tiny bathroom. She gets him to exercise by telling him to touch this wall, no, I meant this wall. Ma notes a bad toothache.
At night, Ma bakes the cake. Due to the limited resources, it ends up being very small and not impressive. Jack asks why there are no candles with fire. She tells him they can’t get them. He is upset and refuses to eat the cake, telling her she could have asked for candles from Old Nick for their Sunday treat instead of jeans like last time. She says she gets things they need for Sunday treat. He says maybe he’ll ask for candles for when he turns six next week. She tells him he turns six next year.
It gets late and Jack goes into a tiny cupboard where he sleeps. Old Nick comes in and we see him briefly from Jack’s point of view, through the slats. Nick asks about the cake and tells Ma if he knew the boy had a birthday, he would have brought a present. Ma’s small bed begins to squeak as they have sex off screen. Jack merely counts higher and higher as this goes on.
The next morning, Jack finds his mom in bed. They go throughout their normal day. She cuts apple slices and her bad tooth comes out. She gives it to Jack who keeps it in his mouth. He watches TV on a CRT television and talks of the TV people, which he thinks of are some form of aliens. Ma teaches Jack, educating him on various subjects. He takes a bath. Jack complains that his mom didn’t tell Old Nick that it was his birthday and that Nick would have brought a present. Ma tells him he was lying. She makes him dinner, which is just a piece of toast. He asks if he can have something else to eat but she doesn’t respond, irritated at her limited resources.
The next morning, Jack wakes up and sees a brand new remote control race car for him. He plays with it all around the tiny room, much to Ma’s annoyance. With the whirl of the race car making noise, Ma begins screaming up at the ceiling, which we see is lined with soundproof material. There are no windows in the room but only one skylight showing the blue sky above. Jack begins telling his mom about his pet dog, Lucky, and how he can’t wait to be reunited with him. His mom tells him he invented Lucky in his head and that they have no dog. Jack sees a mouse in the kitchen and tries to befriend it but Ma scares him away. Jack is upset but Ma explains that the mouse could eat their food or bite them when they sleep. He tells her she didn’t have to kill it and she says she didn’t it went to the backyard with his family. Jack doesn’t know what she means.
That night, Ma is making cheese sandwiches. Jack asks what smells funny and she realizes she left the toaster oven on and it has caught fire. She is worried about this. She sends Jack to bed. He wonders, when he dreams, where he goes. She tells him he is always there, in that room. A padlock code is punched and Jack stays silent in his cupboard. Old Nick comes in with groceries. He asks about the smell and she tells him she burned the sandwiches she was making and she wasn’t thinking. He tells her thinking was never one of her strong points. When she asks him about something missing from the bags, Old Nick gets violently angry with her and says its hard to have to provide for the both of them. He tells her he was laid off six months ago and asks if she knows how hard it is to buy things without an income. She asks if he’s been looking for a job and he gets angry with her. In the cupboard, Jack stirs, watching the argument. Old Nick notices and asks if he’s really asleep in there. He tries to prove he is awake by telling Jack to come out because he has candy but before Jack can take the bait, Ma lures Nick to the bed.
Later that night, Jack sneaks out of his cupboard and tiptoes over to Ma’s bed. Old Nick is still asleep next to her. Jack peers over to look at Nick, who then opens his eyes. He lunges for Jack but Ma wakes up and throws herself on top of Nick. He clobbers her, allowing Jack to escape. Nick storms off and Jack begins sobbing, telling his mom he knows he is not supposed to leave the cupboard and he will never do it again.
The next morning, Jack wakes up in Ma’s bed and notices he can see his breath. When Ma wakes up, he points this out to her, telling his mom he’s a1 dragon. Ma realizes Nick has cut the power. They are in the dark and the heater is off so they are freezing. They bundle up in blankets.
Ma sits Jack down and asks if he wants to know where the mouse went after he left the room. She tries to explain the mouse is outside, on the other side of the wall. Jack asks if she means outer space with the TV people. She says, no, that wasn’t true. He was younger and so she made up a story that he’d understand but now he is five and should know the truth. She tries again, telling him there are two sides to everything. He says, Not an octagon. That has eight sides. She said but every wall has another side and on the other side are buildings and people and trees and a house with a hammock that she used to live in. That when she was 17, a man told her he had a sick dog to lure him into his custody. Jack interrupts, asking about the dog. She screams there was no dog. She tries to explain that there is a world outside the room that he’s lived in his whole life but Jack cannot conceptualize this as he has never seen anything beyond that room. She explains that the TV people are images of real people. He asks if Dora (the Explorer) is real and she says, no, she is a cartoon. But people are real and they’re all outside. He calls her a liar. She tells him the things she said before were a lie and now shes trying to tell an un-lie. He says he wishes he were four again. He suggests she tells him these things when he’s six instead of five.
After some time, the power is turned back on. A leaf falls on the skylight and Ma points this out. Jack doesn’t believe it because leaves on TV are green but she sees they are green on trees but turn brown when they fall off and rot. Ma tells Jack she has a plan to get him out. She boils water on the stove and says she’s going to convince Old Nick that he is sick. He will take him to a place called a hospital where there are people around. Jack is going to give a note to anybody he sees in the hospital and tell them his mommy’s name (since she was sure to have been a high profile kidnap victim). Jack is nervous because he’s never seen a real person before and doesn’t quite know what to expect. He asks why they don’t just hurt Old Nick and then escape. Ma said she tried once, years ago she took the toilet lid off and when he came inside, she tried to smash it over his head. But he caught her and hurt her wrist and that’s why its been in bad shape his whole life. Its about time for Old Nick to come in so she pours the hot water on his face, which makes him scream but she tries to assuage him, telling him he must pretend to be sick and act floppy. She then forces herself to vomit and spreads it around Jack’s pillow, which freaks him out. When Old Nick comes in, Ma pretends to be hysterical, explaining that Jack got sick when the power was turned off and they were left in the cold. Old Nick notes that Jack is burning up and says he’ll get some antibiotics the next day. But Ma insists he takes Jack to an emergency room because it is urgent. He refuses.
The next day, Ma says she has a new plan. Jack will pretend to be dead instead of floppy, he’ll be stiff like a robot. She will roll him up in a rug and Old Nick will put Jack in the back of a truck. He will feel movement and unroll himself in the rug. There will be a long stop and that’s when Jack has to jump out and run to the first person he sees, asking for help. She rolls him in the rug but too tightly so he can’t unravel himself. She tries it again where he is more loosely wrapped and he is able to get himself out. Jack watches some TV and is now able to discern that the people he sees are real people being filmed although the cartoons are, in fact, not real. He is finally beginning to understand.
They are discussing the plan with her repeating over and over that he will unravel, jump out when the truck stops, and then find help. She tells him to picture his moms voice giving him guidance even though they’ll be apart. When he asks her if she will join him in the outside world, she agrees but it is obvious she doesn’t believe she will survive once Old Nick realizes she has tricked him into letting Jack escape. Just then, they hear the padlock and Ma has to be quick to roll Jack up into the rug. She finishes just as Nick enters, telling her he has the antibiotics, but Ma begins crying, saying that he killed Jack when he turned the power off. She begs him to give Jack a proper burial in the woods, some place with trees. Nick agrees and picks Jack up in the rolled up rug.
The rug is loaded into the back of Old Nick’s pickup truck, just as they planned. He drives off to bury Jack in the woods. From inside the rug, Jack feels the truck slow to a stop. He begins to unravel and is successful at releasing himself from the rug. Old Nick rolls through two stop signs, then proceeds over railroad tracks. Jack decides then to stand up and jump out, looking for a person. Nick sees this happening in his mirror, says Stupid bitch in regards to Ma’s tricking him, and stops the car. Simultaneously, Jack begins running towards a real person with a dog. But Nick grabs him and violently tries to return him to the vehicle. Jack is too scared to talk to the person but tries handing him the note as he is dragged away. The man is suspicious and tells him to leave the girl alone (he mistakes Jack as a girl because of his long hair) and that he is going to call the police. Old Nick abandons Jack and speeds off in the truck. Jack is now left alone with the man.
Time has passed and Jack is greeted by a friendly police officer and her cranky partner. He is put into the back of the car and asked questions. The lady’s partner theorizes the boy has probably escaped from some cult but the female officer is more patient with Jack. He shows her Ma’s bad tooth that he keeps in his mouth and finally begins to talk. She asks what his mom’s name is but he can’t remember. She asks him where he lives but he tells her a room. She asks if the room has any windows and he says ‘no’. She asks how they get light and he tells her there’s a skylight. He remembers that it is called a shed. He explains that he was supposed to jump out when the truck stopped but it stopped two times and then went sideways. She tells her partner to put out a radio call to officers to look for sheds with a skylight in the backyards of houses three blocks from the railroad stop. She says a red pickup truck should be in the driveway.
Time passes and the police car shows up in front of the house that we’ve seen when Old Nick loaded the rug into his truck. The officers leave Jack alone in the car and he can’t open the door because it is a police car. Then suddenly Ma comes running around the corner, followed by officers. She rushes towards the car with Jack inside. She can’t get in and Jack can’t get out and she bangs on the glass.
The next scene, Jack wakes up in a hospital room, which by his reckoning feels enormous with large windows overlooking a grander world he’s been isolated from all this time. Ma wakes up and he tells her he wet the bed and he is sorry. She’s not upset with him. She throws away his underwear and tells him they’ll get more. They have a lot of food and he asks if it is for their Sunday treat and she tells him they’re going to have lots of treats and not just on Sunday. He wants to take a bath but she tells them they have a shower stall. They shower and play and he admires the view of the city. He is overwhelmed at seeing more of the world than the room for the first time.
Ma and Jack are treated and given many tests to take. The doctor comes in and suggests Jack wear a mask and sunscreen before he goes out since he hasn’t built up immunity to the environment, but overall he’s still young enough to likely develop normally. Regardless, Jack also has to endure being given vaccinations for the first time to acclimate him properly. It is mentioned that Ma will have surgery on her wrist that Nick damaged after she tried to harm him. Ma tells Jack that his grandma and grandpa are going to meet them. Grandpa now lives several hours away but he has flown in. The two (Joan Allen and William H. Macy) burst in the door, tired of waiting to be admitted, and embrace their long-lost kidnapped daughter.
Jack and Ma are taken to her childhood home where Ma’s mother now lives with her new husband. The media is camped outside the house but the family ignores them. Jack wants to know where the hammock is that Ma always talked about but she isn’t sure; it must have been removed in the last seven years. Jack is told that his grandma’s new husband has a dog but he keeps him in the country. Inside, there are tons of toys for Jack, which Ma explains are from all the people who care about him. He wants to know who these people are. Jack’s grandmother asks if he wants to cut his hair but he whispers his response to Ma that his strength is in his hair.
Ma notices her room is exactly as she left it. She goes through her old yearbook and shows Jack an old picture of her from high school with members of her track team, on which she ran anchor. She asks if he knows what happened to the other girls and Jack said he doesn’t and she said, “that’s right because nothing happened to them.”
At dinner, Jack is given ice cream and experiences his first brain freeze. Ma’s mom and stepdad try to make conversation but Ma gets violently angry with her own father, demanding that he looks at his grandson but he can’t because he is too uncomfortable (i.e., because Jack is the product of his daughter being raped by her abductor). She finally takes Jack upstairs.
Ma and Jack have a hard time settling into life outside their captivity. Jack asks Ma when they are going to leave but she tells him they live there now. He is still wary of people and constantly hides in small spaces. Ma’s stepdad is very friendly to him, which makes him a little more comfortable.
The family’s lawyer comes and tells them that there is going to be a trial and they will need money and the easiest way to get some is to do a prime time interview.
A broadcast journalist shows up at the house. Ma is made up by the crew and prompted that if she feels uncomfortable with any questions, she can simply request they move on. The interview begins and the journalist tells Ma it must have been hard to maintain her faith in God after all those years in captivity and asks if she ever considered taking her own life. This derails Ma who doesn’t answer. The journalist then asks what Ma will tell her son when he asks about his father one day. Ma responds that Nick was not her father to which the journalist inquires if there were other men. Ma explains, no, but he is not Jack’s parent she is his only parent. The journalist doesn’t acknowledge this response and instead says that he is his father, biologically speaking. She asks another invasive line of questioning: why didn’t she have her captor take Jack away when he was born? The journalist suggests Jack could have been dropped off at a hospital and been able to live a normal life. She then suggests Ma only kept Jack imprisoned out of a selfish desire to not be alone.
The interview has upset Ma and she sinks into a depression. She is aggressive with Jack and tries to force him to play with the toys he’s been given by the sympathetic masses. Jack isn’t interested so she forces him to play with Legos. Mas mother tries to defend Jack but Ma explodes on her mother, saying she seems to have been able to continue on with her life just fine in Ma’s absence. Ma’s mother encourages her to be nice but she said her mom’s voice telling her to be nice is what ran through her head when a man approached her about a sick dog.
Later, Jack hears some noises coming from the bathroom. He enters to find his mom unconscious on the floor. She has swallowed pills in a suicide attempt.
Ma is taken away. Jack is left alone with just his grandma and her boyfriend. But he begins to get more acclimated with the world and feel more comfortable in their home. His grandma’s husband even brings his dog home, which excites Jack, who still remembers his (imaginary) dog, Lucky.
Ma calls Jack from the hospital. He doesn’t know how to use a telephone but they have a short conversation.
Jack tells Grandma that he wants her to cut his hair (which has never been cut in his whole life). She asks if he’s sure and he says it is his strength but now he wants to send it to his mom so it can be her strength. His grandma tells him she used to cut his mom’s hair and then cuts off his ponytail in a few snips which he is worried about, expecting it to hurt. She tells him she has wanted to cut his hair for a long time. He tells her, I love you, Grandma.
Time goes by and a neighbor spies Jack through the window. Jack goes into the backyard and begins to play with him. It is revealed that Ma is watching from inside. She greets Jack and tells him that when he sent his hair, it made her happy and gave her a reason to come home. Ma and Jack play Legos together, their camaraderie from early in the film returning. Jack asks Ma if they are ever going to return to the room. She is taken aback by this and he tells her he means just for a visit.
Ma and Jack return to the shed which now has the front door blasted off (how Ma was initially rescued). It looks completely different from his new perspective so Jack asks if its been shrunk. All of the furniture is gone which Ma explains is because the police took it for evidence to prove that they were there. Jack tells her its different with the door open. Ma asks him if she wants him to shut the door but he says, no.
Just like when he said hello to everything in the room, Jack now says “Bye, Chair #1. Bye, Chair #2. Bye, Cupboard.” He then tells Ma to say goodbye to Room. Privately, Ma mouths “Bye, Room.”
Ma and Jack walk away from the shed with police escorts in the distance, who have given the two the opportunity to say goodbye to their prison for their years of captivity.