Zach Cooper and his mother Gale move to the town of Madison, Delaware. While settling in, Zach meets his new neighbor Hannah and her father, known as “Mr. Shivers”. Zach and Hannah become quick friends, but Mr. Shivers frequently warns Zach to avoid contact with them. At Madison High School where Gale works as the vice-principal, Zach befriends Champ, a socially awkward student. The next night, Zach hears Mr. Shivers and Hannah arguing, followed by Hannah screaming. He calls the police, but Mr. Shivers assures the officers that the noise came from his television. Fearing Hannah is in danger, Zach tricks Mr. Shivers into going to the police station for further questioning and breaks into his house with Champ’s help. The two find a shelf containing many Goosebumps manuscripts, all of which are locked. Zach unlocks one of the books, but when Hannah discovers them, Zach drops the book, releasing the titular character of The Abominable Snowman of Pasadena that breaks out of the house. They pursue the monster to an ice skating rink, where Mr. Shivers appears and imprisons it back into the book.
On the way home, Mr. Shivers reveals that he is Goosebumps author R.L. Stine. He created the stories as a child to terrorize the people that made fun of him, but the monsters came to life and became uncontrollable, so he had to trap them inside their manuscripts. Returning to the house, they find Slappy from the Night of the Living Dummy, which was accidentally unlocked before, waiting for them. Angry at being imprisoned, Slappy burns his own manuscript and flees in The Haunted Car. They are attacked by the lawn gnomes from Revenge of the Lawn Gnomes, but when Stine and the kids try to smash them, each of the gnomes reform, making them realize that they can only run away.
Slappy releases several monsters, letting them rampage under his command. Zach suggests Stine to write a new book that will trap every monster, but he can only do it with his special typewriter which is at the high school. On the way there, Brent Green from My Best Friend Is Invisible and the giant praying mantis from A Shocker on Shock Street attack Stine’s car, so they are forced into hiding. The titular Werewolf of Fever Swamp pursues them into an abandoned supermarket, only to be run over by Zach’s aunt Lorraine.
As they cut through the cemetery, Zach notices that Hannah glows blue in the moonlight. After evading the Graveyard Ghouls from Attack of the Graveyard Ghouls, they manage to make it to the high school. There, Zach privately questions Stine about Hannah, and Stine reveals that she is a Goosebumps character (from The Ghost Next Door) he created to cope with his loneliness, of which she is seemingly unaware. Stine finds the typewriter and starts writing a new story based on the events around them, while Zach leads the students in fending off all the released monsters that are storming the building. Slappy confronts Stine and breaks his fingers with the typewriter case before the story is finished.
Stine, Zach, Champ and Hannah board a school bus and go to an abandoned amusement park. There, Slappy lets out the blob from The Blob that Ate Everyone. Stine then hands the book and typewriter to Zach, and distracts the blob by allowing it to devour him alive. Zach finishes the story, but is reluctant to open the book because Hannah will also be sucked inside it. Revealing she knew the truth about herself all along, Hannah opens the book, sucking Slappy and all the other monsters into it; she bids farewell to Zach with a kiss before she is also sucked in. Sometime later, Stine has started working as Zach’s English teacher. After class, Zach is surprised and delighted to see that Hannah is there and Stine reveals to Zach that he wrote her back into existence with another book. She and Zach kiss and leave school together while Stine burns Hannah’s manuscript, making her permanently real. As he prepares to leave the school, he sees the typewriter typing by itself inside its display case. To Stine’s horror, Brent Green has evaded imprisonment and begins writing a story titled The Invisible Boy’s Revenge.
It’s a fun story even if you are not aware of Goosebumps, but I’m sure if you are a huge fan, you’ll get a real kick out of seeing some of the monsters from your favorite books/TV epoisodes make an appearance on the big screen.