*CUT TO THE CHASE*
This spoiler was submitted by Jeremy
In the town of Wardenclyffe, high school senior Sarah Quinn (Madison Iseman) is struggling to write a paper to send to Columbia University so she can get in. Her boyfriend Tyler Mitchell (Bryce Cass) sneaks in through her window to hang out, but they are heard by Sarah's mother Kathy (Wendi McClendon-Covey) and brother Sonny (Jeremy Ray Taylor), and Kathy makes Tyler go home.
The next day, Sonny's best friend Sam Carter (Caleel Harris) is dropped off to stay for the weekend. Kathy stops at a store on the way to taking the kids to school, and she appears to be having a connection with the store's manager Walter (Chris Parnell).
At school, Sam puts up flyers for his and Sonny's unofficial business of picking up other people's junk. They are picked on by bully Tommy Madigan (Peyton Wich) and his two goony friends. When Sam sticks up for him and Sonny, Tommy threatens to torment them.
Kathy orders Sarah to look after the boys because she is working late, but Sarah plans to leave the boys to go out with Tyler. At the same time, Sam gets a call from someone asking to pick up some stuff from a house. He and Sonny go to the house, which looks old and abandoned. The boys pick up a bunch of random junk until Sonny finds a stuffed cat and pries it off the fireplace, which opens a secret compartment containing a chest. Inside is a locked book. The boys open it, and when they turn their backs, they don't see that Slappy the dummy (Mick Wingert) has appeared. Sonny finds a note in his shirt pocket, and he reads the incantation that brings Slappy to life.
As the boys leave the house, Tommy and his friends find them and try to harass them. Sam refuses to give up his stuff, so Tommy swipes Slappy from their hands. The dummy uses his powers to pull down Tommy's pants repeatedly. The boys get their stuff and ride away on their bikes before the bullies pursue them. Slappy makes a hose come out and clothesline the bullies.
Later that night, Sonny is working on a presentation on Nikola Tesla for his science class, which he practices using Slappy and other dolls. Slappy then speaks to Sonny, who slowly realizes the dummy is alive. Sonny shows Sam, and Slappy demonstrates his powers on Sonny's toys to show that he can bring things to life, but he wants them to keep it a secret for now.
Sarah goes to meet Tyler at the show that he invited her to, only to see him kissing another girl. She goes home upset and complains to Sonny about it, which Slappy overhears.
At school, Slappy sneaks into Sarah's backpack, so she stuffs him in the locker. He overhears as Tyler talks to Sarah, and she is still mad at him. Slappy finds Tyler in the gym as he is setting up decorations, and Slappy scares Tyler by causing the ladder he is standing on to collapse, injuring the boy. He is wheeled into an ambulance raving about Slappy, which Sarah overhears. Meanwhile, in Sonny's class, he presents his Tesla Tower project to an impressed class, until it goes out of control and shoots electric bolts that wreck the science lab.
Sarah confronts Sonny about Slappy, and he admits to her that the dummy is alive. They go home to find Kathy playing with Slappy. When she leaves the room, Slappy speaks to the kids, which freaks Sarah out. Sonny reads the incantation again to make Slappy inanimate again, but it doesn't work. The three try to talk in private in Sarah's room, but Slappy appears there as well. Sarah grabs an aluminum bat and knocks Slappy out. They put him in a suitcase and drive to a nearby lake to dump him there, but he reappears on the windshield as Sarah drives them home. She speeds up and hits the brakes hard to throw Slappy off, but Sonny fears they have just made things worse.
The kids go home to read up on the events of the first movie in Madison, DE where creatures from the books of R.L. Stine (Jack Black) terrorized the town, including Slappy. They try to get in touch with Stine but find a different number and leave a message, which Stine hears on his machine. Knowing something bad is going to happen, he makes his way toward Wardenclyffe. They then figure they need to get the book back, but Tommy took it, so they need to get it from him.
Slappy walks into town and goes to Walter's store where he finds a bunch of Halloween decorations, including the Abonimable Snowman and Werewolf of Fever Swamp. Using his powers and the incantation, Slappy brings the scariest decorations to life and throws a mask onto Walter to turn him into a monstrous henchman. The creatures head off into town to wreak havoc.
The kids find Tommy and his buddies TP-ing someone's house when they go to ask him for the book. He refuses to give it back until Sarah forcefully demands that he give back the book. Before anything else happens, three witches swoop down and take the bullies into the air.
The kids then go to Tommy's house while his grandma is asleep to try and find the book. Sarah stays outside to keep watch while the boys sneak inside. As this happens, Slappy and Walter make it to the actual Tesla Tower in town so that Slappy can get it running again and animate every last Halloween decoration in town. Sonny finds the book, but he and Sam are attacked by gummy bears that grow large and try to absorb the boys. Sonny opens the book, which sucks the gummy bears inside. Realizing the book's power, the kids head out into the night to stop the other monstrosities. Sarah looks at the book and sees that it is unfinished, but it ends on Slappy wanting a family, specifically a mother. They rush home after Sonny tells Sarah that Kathy called and she is on her way home to check on them.
Near their home, the kids find other townsfolk being terrorized by the monsters. Kathy has been strung up by a spider web from a giant spider made of balloons. Sarah opens the book to pull more of the monsters inside, but a ghost flies by and takes the book away from her, and the spider takes Kathy away as well. The monsters start to close in on the kids until they get pulled into the home of their holiday-obsessed neighbor Mr. Chu (Ken Jeong). He gets excited when the kids tell him that the monsters are from "Goosebumps" stories, but they make it clear how urgent it is that they get the book. Mr. Chu helps the kids sneak out by dressing them up to look like monsters.
Stine arrives in town in the middle of the chaos and recognizes this from his very first book, "Haunted Halloween". He also notices a familiar red balloon floating over a sewer and brags that he came up with that first.
The kids make it to the Tesla Tower to find that Slappy has turned Kathy into a dummy. Sarah goes after Slappy to get the book from him while the boys fend off the monsters that are closing in on the tower. Sarah reaches the top and fight Slappy, who loses the book in the struggle. Sarah tries to bargain with Slappy to turn her mom back to normal if he wants the book back. Slappy agrees, and Sarah tosses him the book, only it's a copy of "Frankenstein" that she took from Mr. Chu's place. She kicks Slappy into the electrical coils, which launches him into the sky. She opens the book and sucks all of the monsters all over town inside the book, and Kathy and Walter turn back to normal, but they have no memory of their mutations.
Stine arrives just as the group is leaving the building, and he sees that the kids fixed the problem themselves, but they give him back "Haunted Halloween". Before he goes back home, Sarah asks Stine for tips on being a better writer, and he gives her some helpful advice. Tommy also falls from the sky and thinks he just had a crazy dream, but he hugs Sonny and is happy to see him.
Some time later, Sarah finishes her essay and submits it. Later, she gets an email from Columbia stating that she has gotten in.
Back in Stine's home, he puts "Haunted Halloween" away along with his other manuscripts. That's when Slappy appears behind him, not having been sucked into the book. He reveals that he wrote a book of his own, with Stine as the main character. Slappy opens the book and pulls Stine inside as the dummy laughs maniacally.