The year is 2016 and Amanda is a thirty-something mother to her 13-year old daughter, Erin. Her husband, Jim, has recently left her, claiming she is emotionally unavailable. It's Halloween, and Erin comes downstairs dressed in a rainbow wig, silver cape, bandit mask, and blue face paint — the costume of "Hannah-beast." Hannah-beast is a local legend of a girl who died some thirty years ago, now a favorite Halloween costume among children. Amanda, a shiver going up her spine, refuses to let Erin leave the house in the costume, and makes her change into a cat outfit. Erin asks her mother for her rationale, but Amanda says nothing. Erin is furious and swears at her mother, before leaving with friends. Relations are tense between the two, especially after Jim has left, and Erin has begun swearing and becoming more rebellious.

The narrative flips between 2016 and 1982, when the Hannah-beast incident occurred. It is revealed that Amanda was part of a trio of middle school bullies, along with ringleader Mel and her sidekick Katie, and they often bullied a girl named Hannah. Hannah lives with her father, implied to be an alcoholic, and faces extreme bullying at school. The three girls have pulled several pranks on her throughout the years, including offering to shave her legs, and then cutting her intentionally. Out of the three of them, Amanda is the most compassionate towards Hannah, and even once has a sleepover with Hannah. During the sleepover, Amanda gives Hannah her old clothes, including a pair of pink cowboy boots. Hannah is very affectionate towards Amanda, and gives her the nickname "Manda Panda." But the next day at school, Hannah wears the old clothes and Mel and Katie accuse her of stealing them and force her to take them off. In front of Mel and Katie, Amanda does not defend Hannah and goes along with them.

The three girls devise a complex plan to humiliate Hannah. They convince Hannah to come out with them on Halloween, and cobble together a costume for her using parts of their own costumes: a rainbow wig, a silver cape, a bandit mask, and blue face paint. They call her Hannah-beast, and when they trick-or-treat, Hannah yells "Boo!" at all the adults, startling them. The foursome gather candy, and then tell Hannah they have one more surprise for her: a scavenger hunt. They give her a series of clues and tell her they will meet her at the end.

The clues take her to a shed, where she must steal a crowbar, then to a garage, where she pries open the door and steals a can of gasoline, then to a barn where she must steal a lighter. In the barn, she must burn all the paper clues. After burning them, Hannah hides behind a tree to wait for the others, but realizes the barn is on fire. The barn burns to the ground, and in it, two young children who have a tradition of sleeping in the hay loft every Halloween. Their parents are devastated, and want to find the culprit. At that moment, Mel, Katie, and Amanda emerge and point to Hannah, who is standing behind a tree with a can of gasoline and a lighter, obviously framed. Hannah runs away to the gazebo, where she hangs herself using a rope from a piñata. It is revealed that Mel set the fire, intending to blame it on Hannah, though didn't realize two children were sleeping inside.

In the present day, Amanda hands candy to trick-or-treaters and is triggered by how many of them are dressed as Hannah-beasts. She drinks a whole bottle of wine and decides to carve a pumpkin to get in the spirit. While carving, she accidentally cuts herself and starts bleeding. She tries to carve a happy pumpkin, but once it's done, she realizes it looks exactly like Hannah-beast come back to haunt her. Drunk, she is haunted by flashbacks and feels deeply remorseful for her cowardice at not standing up to Mel or defending Hannah all those years ago. Suddenly, the power goes out and she hears noises in the house. She looks in the mirror and sees a Hannah-beast behind her and starts crying. Finally, the Hannah-beast looks up to her: "Mom?" It's revealed that it is her daughter, Erin.