Zach Cooper, portrayed by Dylan Minnette, finds himself reluctantly relocating from the bustling streets of New York City to the quieter town of Madison, Delaware, alongside his mother Gale, played by Amy Ryan. Upon settling into their new home, Zach encounters a captivating neighbor named Hannah, brought to life by Odeya Rush. However, their initial introduction is interrupted by Hannah's father, R. L. Stine, portrayed by Jack Black, who sternly advises Zach to keep to his side of the fence.
In the solitude of his new room, Zach watches a sentimental tribute video honoring his late father, a fireman who heroically lost his life in the line of duty. The next day at school, where Gale holds the position of vice principal, Zach faces an assembly where his mother speaks of the upcoming fall dance. Throughout this awkward introduction, a fellow student named Champ, played by Ryan Lee, attempts to engage Zach in conversation, leading to an unexpected friendship.
That evening, while disposing of garbage, Zach is startled by Hannah and learns of her homeschool status. The two share a moment as they venture into an eerie forest, discovering an abandoned theme park. Together, they scale the towering Ferris wheel, where they share personal stories and secrets—Zach talks about his father, while Hannah reveals that she has been uprooted to different towns due to her father's mysterious nature.
However, Zach's curiosity leads him to fear for Hannah's safety when he hears a scream and is rebuffed by Mr. Stine, who claims Hannah is in London with her mother. Concerned, Zach calls the police, but they find nothing amiss. Later that night, as Gale babysits, Zach sneaks into the Stine household, where he uncovers a series of peculiar items, including locked Goosebumps books. His intrusion awakens the terrors within—upon unlocking "The Abominable Snowman of Pasadena," a monstrous snowman bursts forth, setting off a cascade of chaos.
Zach, Hannah, and Champ find themselves in a race against time to manage the increasingly bizarre occurrences, which include escapades at an ice rink and dangerous confrontations with relentless monsters. With shocking revelations about Mr. Stine's past, they discover that the creatures he conjured in his writings can become tangible threats when unleashed. Stine explains that as long as his books remain locked, they're safe, but the emergence of a living dummy named Slappy upends their fragile safety.
As monstrous mayhem grows, including a rampaging praying mantis and a legion of gnomes, the group rallies together at the school, using their wits and bravery to forge a plan. In the midst of the turmoil, Zach learns that the only way to control or eliminate the monsters is through Stine’s typewriter, a portal to the stories that birthed their nightmares.
With the responsibility to recapture the monsters before they wreak irreversible havoc, Zach leads his friends in a thrilling pursuit, culminating in a climactic battle at an amusement park. As the danger reaches a fever pitch, friendships are tested, and Zach must make a heart-wrenching choice to protect the ones he loves. In a poignant moment, he bids farewell to Hannah as she fades away, a testament to their profound connection and the adventures they've shared.
Months later, as the town recovers, Zach has emerged as a popular student, and a new teacher enters their lives—surprisingly, it is R. L. Stine himself. Reflecting on their hair-raising experiences, Zach is reminded that while he might miss Hannah, her spirit lives on in their memories and the pages of countless tales yet to unfold. As Stine discovers his typewriter mysteriously active once more, the story hints at new adventures ready to come to life.
