Peter is sleeping in his room when he suddenly hears a strange sound coming from the wall. He doesn’t think it’s anything serious, so he just continues trying to sleep.
However, the sound becomes louder, and Peter wakes up again, looking scared. The sound continues, and out of fear, Peter gets up from his bed and stares at the wall where the sound is coming from. Still wondering what could be making the sound, Peter goes closer and bangs on the wall.
Surprisingly, he hears a similar bang from the wall in return. He runs out of his room and silently enters his parent’s room. Peter wakes his mother, Carol, and tells her that he’s hearing sounds coming from the wall. While Carol tries to understand what Peter is saying, Peter’s Dad, Mark, also wakes up.
As Peter seems terrified, Carol goes with him into his room to check what’s possibly wrong. Peter points to the wall he heard the sound from, and Carol goes closer. However, she doesn’t hear anything. She then tells Peter that since their house is old, it’s likely for him to hear sounds from the wall.
The next morning, the school bus arrives to take Peter to school. As he enters the bus, he is tripped by a classmate named Brian. Despite seeing Peter fall to the ground, Brian tells Peter he’ll trouble him more during recess.
When they get to school, Peter is the only student not playing around just before the principal walks in. He introduces a new substitute teacher named Miss Devine, who will now be in charge of the class. Miss Devine seems genuinely excited about teaching the class, and her introductory speech proves that. During recess, Miss Devine starts to decorate the class for Halloween, which is in one week’s time.
Despite all his mates already out to recess, Peter stays back, likely fearing to get bullied by Brian again. Miss Devine sees that Peter is the only one in the class, and she looks concerned. She asks him why he’s not joining the others in recess, but he says he doesn’t like it. Miss Devine tells Peter it’s not a problem and he can stay in the class and help her with the decoration.
She asks Peter what he’s going to be for Halloween, and when he says he doesn’t know, she tells him there’s still a lot of time to know. As they talk, Peter suddenly gets startled by a spider on his table, but Miss Devine helps him get rid of it and feel more comfortable.
When he gets home that day, Peter asks his parents if he can dress up for Halloween. He talks about how he wants to experience it like the other children in his school. His mother doesn’t sound happy with this question and tells Peter not to even think about it.
Seeing as he’s sad about Carol’s response, Mark tells Peter a story about a girl who went missing a few years back on Halloween day, in the same neighborhood they’re staying in. This gets Peter scared, but Carol and Mark assure him that they’ll make sure he doesn’t go missing. At night, while he’s sleeping, Peter suddenly hears a voice from the wall calling his name. This gets him very scared, and he shouts for Mark to come.
As Mark enters the room, Peter tells him that he heard something again. Mark doesn’t seem to believe this, as he tells Peter that he’s got a really wild imagination. To make Peter feel more comfortable, he goes close to the wall to see if he can hear anything.
After a while of not hearing anything, Mark says that if there’s anything there, then it’s rats, and he’s got something to deal with them. Together, Mark and Peter head over to the storeroom, where Mark shows Peter the rat poison he plans to use. Peter picks it up, and as he smells it, he says it smells like cinnamon. Mark tells him that he can use that as a lesson that not everything is as sweet as it seems.
Mark starts putting the rat poison on the floor when they get back in, and Peter is curious to know what will happen to the rats after they eat it. Mark tells him the rats will die, and Peter looks very sad. Seeing this, Mark convinces Peter that having rats in the house is bad and can lead to diseases.
He also mentions that they might be making sounds that will keep Peter from sleeping. Peter still looks sad, and Mark tells him that sometimes people have to make tough decisions to protect their family.
The next day, Miss Devine is reading a scary story to the class, while the kids are drawing something for Halloween.
As she passes Peter’s table, she sees him drawing something quite troubling. The drawing is about someone who is in a room and calling for help. Miss Devine is concerned about this, and later that day, she goes to Peter’s house.
She knocks on the door, and Carol answers it. After introducing herself, Miss Devine asks if Peter is okay, and Carol says nothing is wrong with him. She then shows the drawing to Carol, who then plays it down as nothing serious. Carol says Peter has an overactive imagination, which sometimes makes him do wild things.
After Miss Devine leaves, Carol takes the drawing to Peter to get an explanation. Peter says it’s just a drawing, but Carol sees it as Peter asking a stranger for help. Peter continues to say it’s only a scary drawing and it’s nothing serious. Before she leaves his room, Carol tells Peter that his wild imagination might get him in trouble one day.
That night, Peter again hears the voice from the wall. The voice sounds like that of a young girl, and she tells Peter that she just wants to talk and be friends. Peter responds, saying he’s only imagining her voice. However, the girl says she’s real, and if Peter doesn’t want a friend, she can stop trying to talk to him and leave. Peter feels guilty for sounding harsh, and he agrees that whoever it is can be his friend.
The next day, Peter is painting his pumpkin at school, when Miss Devine comes closer to commend him. She secretly tells him that his painting is her favorite. From where Brian is seated, he looks jealous of Miss Devine’s special interest in Peter.
Later that day, when school closes, Peter still looks happy about Miss Devine’s comments as he carries his pumpkin. Out of nowhere, Brian shows up and bullies Peter again. In the process, he also crushes Peter’s pumpkin. Peter looks sad and starts crying in his room when he gets home.
As he needs a friend in such a moment, the voice from the wall starts comforting him. Peter tells her about Brian, and just then, the voice starts giving Peter some really terrible advice about how he needs to show Brian that he’s not to be bullied.
The next day in school, Brian brings in a horribly painted pumpkin to give Peter to serve as an apology. However, the drawing is not good enough, and the students in the class laugh about it.
Peter starts remembering the things the voice from the wall told him to do, and also how Brian will never stop if he’s not taught a lesson. After class, Peter follows Brian and pushes him down the stairs. Brian falls badly, and he breaks his leg.
Following this, Carol is invited by the school principal concerning what Peter did. After a moment of arguing that Peter probably didn’t mean to push Brian, it doesn’t seem to be enough as Peter gets expelled.
At home, Mark is very angry with Peter. He tells him that they’re not a family that solves problems with violence. Peter tries to make a case for himself by saying he didn’t mean to hurt Brian. But just then, Carol mentions the drawing that made Miss Devine pay them a visit.
Mark doesn’t understand why he’s just hearing about it, and this makes him angrier. He tells Peter he’s grounded, and as Carol tells him to go to his room, Mark tells her that since he’s seriously grounded, then he has to go to the basement instead.
Just as Peter wonders where the basement is, Carol pulls the fridge to the side, and there happens to be a door there. She opens it, and Peter is taken down the stairs and then asked to sit on the floor.
Mark tells Peter to use the time he has alone to think about what he did. After Mark goes back up and locks the door, Peter calls for the voice, but he doesn’t get any response, with this then allowing him to start crying.
Later day, Miss Devine is approached by the school principal, who has learned about her visit to Peter’s house. Miss Devine says she only went because she thought Peter needed help. He says it’s not for her to get invested in the personal lives of students, because it can get bad.
Despite this, Miss Devine tells the principal that something just seems odd about Carol. The principal says she might only appear weird or protective, because a child disappeared in their neighborhood a while ago on Halloween day.
On the next day, while grading her students’ math tests, Miss Devine sees Peter’s test paper and writes her phone number at the top. Even though Peter has been expelled and she’s no longer his teacher, Miss Devine goes to Peter’s house again.
This time, she tells Carol that she’s there to give Peter his test paper. Carol is grateful for that, but just as Miss Devine starts asking if she can see Peter and if he’s fine, Mark shows up. He asks Miss Devine to come in, and she does. Mark appears to be holding a hammer as he welcomes her in.
As he offers her water, Miss Devine sees that Mark is bleeding. Mark says it’s nothing serious and that he must’ve gotten injured while he was doing some remodeling in the house. Miss Devine then starts to ask about Peter again, including where he would now go to school.
Mark says Peter will now be homeschooled by Carol, who used to be a teacher. As Miss Devine continues to talk with Carol and Mark, Peter hears that Miss Devine is around. He starts shouting for help and also makes banging noises to get Miss Devine to hear.
However, the sound from the washing machine is too loud for anyone to hear Peter. Suddenly, Miss Devine says she wants to leave. As she walks towards the door, she finally hears the banging in the basement after the washing machine stops.
On hearing it, she turns to ask what is sounding, and Mark tells her it’s the washing machine. Miss Devine knows that something is off, but she looks scared of Mark. She heads out and gets into her car, but she soon realizes that she forgot her car key inside Peter’s house.
Before she can even think of what to do, Mark appears and gives it to her so she can leave immediately. After Miss Devine leaves, Carol bakes some cookies and takes them to Peter before then asking him to leave the basement.
That night, when he’s in bed, Carol and Mark say they’ve forgiven Peter, after which they leave. Immediately Peter sees that they’re gone, he goes to the wall in his room and tells the voice to please talk to him.
The voice responds to him, and starts telling him things about his parents not being what they appear to be. Peter says he wants to see her, but she says she looks very different, and he will get scared of her and shout.
Peter says he won’t do that, and then the voice asks him to remove a part of the wallpaper from the wall where the voice is coming from. There is a small hole in that part, and from there, Peter can look through to see who it is.
The person behind the wall seems to hesitate, but as she shows one eye, Peter is scared and cannot believe how strange it looks. As he moves back and asks who she is, the girl says she’s his sister.
She tells him their parents locked her in there, and that they’re evil people who will also lock him up soon. When Peter says she’s scaring him, the girl says he needs to be scared because that will give him the courage to free her. His sister says she has been waiting for him to be big enough to move the clock in the living room that blocks the door to where she’s locked.
Meanwhile, she mentions that if he doesn’t believe her, he should dig a part of the backyard and see what his parents are hiding. As that is just too much for him to process, Peter goes to bed feeling scared.
That night, he even has a terrible nightmare where he sees his parents as monsters. When he wakes up, Carol is there to comfort him. She asks him what he saw in the nightmare, but Peter says he doesn’t remember.
In the morning, while his parents are distracted, Peter goes to the backyard. After digging for a while, he sees a human skull buried there. Despite being shocked to see this, Peter immediately has to cover it back up before Carol can see what he did. When Peter returns to his room, the voice tells him that the skull belongs to a trick-or-treater, whom their parents had killed when she asked the person for help.
Peter says he now understands how she feels, and he’s going to do everything to get help and get her out of there. The voice asks where he’s going to get help, and Peter brings out his test paper and says he knows just who to call. He sneaks into his parents’ room and dials Miss Devine’s number.
When she picks up the call, he tells her that he needs help. Before he can say more than that, Carol finds him and collects the phone from him. She then speaks to Miss Devine and says that Peter only wanted to say hello to her, because he misses his teacher. Carol also mentions that Peter couldn’t continue the call because he is shy. Miss Devine doesn’t believe this as she knows that something is definitely wrong.
Angry at what Peter did, Carol enters his room to scold him. As she does, she notices the wallpaper that has been removed from the wall as well as the hole there. She suddenly looks terrified and is visibly shaken as she walks out of Peter’s room.
Carol then asks Peter what the girl has said to him, but Peter doesn’t say anything. Carol says it doesn’t matter and that whatever happens now will be his fault, because she will tell Mark when he gets home.
That night, while Mark and Carol are arguing about what happened and what they’ll now do, the voice tells Peter that they have to act as soon as possible before his parents kill them.
Peter later goes down to listen in on his parents’ conversation. As he does, they see him, but he runs to his room. Mark then goes to Peter’s room and tells him that there’s something they’ve got to do in the garden the next day.
On the following day, Mark takes Peter to the backyard and shows him some of the infected pumpkins in the garden. He tells Peter that they have to get rid of the infected ones to save the good ones. With that, he forces Peter to start digging.
Meanwhile, Brian shows Peter’s house to his brothers, who are willing to take revenge for what Peter did. They take a look at the house and confirm that they’ll be back at night to beat up him.
That night, while the family is having dinner, a trick-or-treater comes to ring the doorbell, but Carol yells at the child to leave. Peter refuses to touch his food even as Mark tells him to eat.
As he continues to eat, Mark notices that the soup tastes different, so he asks Carol if she has used a different recipe. Carol takes a few spoons and realizes that it tastes different.
As Carol continues to figure out what’s different about the soup, Mark figures out that it smells like cinnamon. Suddenly, Mark realizes that Peter has poisoned them with the rat poison.
He immediately tells Carol to call nine one one, after which he starts asking Peter if he did anything. Carol finds out that the landline’s wire has been cut, and just as Mark presses for an answer, Peter says they hurt his sister.
Before Mark can do anything, he starts vomiting a black fluid before he then falls to the ground and dies. Seeing this, Carol picks up a knife and starts to chase Peter. As she catches him, the poison also kicks in, and she starts vomiting before falling down the stairs and dying.
Peter goes up to his room, and it appears that the girl behind the wall had given him the idea to do all that he did. She then asks Peter to get the keys to the door and free her. Peter takes the key and manages to open the door to where his sister is. However, the girl’s voice changes, and she starts laughing weirdly, with Peter running to hide in his room.
Coincidentally, Brian and his brothers arrive at the house. Peter’s sister hears them ring the bell, but she first clears up the bodies of Carol and Mark. She then proceeds to let Brian and his brothers into the house. Suddenly, she appears to be a monster with really long hair, and she kills them one after the other.
Elsewhere, Miss Devine is still troubled by the call she received from Peter, so she decides to go to the house to check on him.
Back at the house, Peter’s sister is now left alone with him. From where he’s hiding, he hears her talking about how she was born a monster, and since then, their parents hated her. She also claims responsibility for the disappearance of the young girl from the neighborhood who she had killed.
His sister also mentions that she has been spending the time in her cage preparing for when she would be freed by him. Shortly after, she finds Peter and locks him in her cage.
Meanwhile, Miss Devine arrives at the house and goes in after no one answers the door. After a while of walking around the house, Peter hears Miss Devine when she calls his name. He immediately shouts for her to leave, but she doesn’t understand why he’s locked in the basement, with dead bodies also around the house. She, however, says she’s not leaving him there.
Out of nowhere, Peter’s sister starts heading toward the room, but as Miss Devine hears the sound, she locks the door. Peter tells Miss Devine that the monster is his sister, and as the creature heads for the basement, Miss Devine breaks the wall to get Peter out.
As they then run out of the house, the monster manages to grab Peter just as Miss Devine runs out. She locks Peter in a narrow cage, and goes down there to talk to him about how she has always been treated unfairly by their parents. Just then, Miss Devine comes in again, and the monster decides to get rid of her once and for all.
The monster manages to injure Miss Devine quickly, but as she’s about to kill the teacher, Peter pulls the monster’s long hair from inside his cage. He eventually uses it as a rope to get out of the cage, and as the monster is powerless and in a lot of pain from the hair pull, Miss Devine starts beating it up.
Nothing seems to happen to the monster, so Miss Devine and Peter push it into the cage and lock it there. As they are about to leave the house, the monster starts to threaten Peter about leaving. She says Peter is no different from her, pointing out that he killed their parents. But Peter yells angrily and says they’re nothing alike.
The monster then says the cage is not enough to keep her down there. She also says that Peter will see her wherever he goes, and she’ll always be right behind him.