Rhode Island teenager Kelly Ferguson, often called "Monster Girl" due to an attack by a monster in her childhood, finds herself in a whirlwind of chaos one Halloween night. Although she had plans to attend a senior Halloween party, her mother convinces her to babysit her son Jacob Zellman, leaving Kelly with strict instructions from Ms. Zellman about what to do and what to avoid. When Kelly arrives, Jacob shares his fears of monsters, showing her his drawings as evidence of their hauntings.
Determined to help him, Kelly reassures Jacob that he will stop dreaming of them just like she did. To ease his fears, she checks the nooks and crannies of his room for any lurking monsters and even provides him with a small light to fend off unseen threats. However, her world turns upside down when she discovers that Jacob has been snatched away by three tiny monsters known as "Toadies." After a failed call to 911, Kelly encounters Liz LeRue, portrayed by Oona Laurence, a member of a clandestine group of babysitters dedicated to protecting children from the supernatural. Together, they embark on an urgent mission to rescue Jacob from the clutches of the Boogeyman.
Liz, employing her skills, utilizes an infant in her care to lure the Toadies, but the shiny object she offers fails to attract them. They transport Jacob to the Boogeyman, who reveals his sinister plan: to harness Jacob's unique imagination to gather an army of monsters. He straps Jacob to a bed equipped with a nightmare-extracting helmet, attempting to sedate him. Following Liz's return of the baby, she and Kelly venture to Brown University, home of the Rhode Island Order of Babysitters. As the vice president, Liz instructs the trainees (SITs) to extract the contents of a captured Toadie and scour for clues about Jacob's whereabouts.
Utilizing a magical dust to recover lost memories, Kelly suddenly recalls a chilling message from the Boogeyman during her childhood: she possesses the power to manifest her dreams into reality. It becomes clear that this Boogeyman is none other than the Grand Guignol, a figure known to siphon dreams and disseminate nightmares. The SITs reveal their own scars from past encounters with such boogeymen, inspiring them to concoct a powerful remedy to combat the Grand Guignol — the Angel Hair potion, which Curtis aids them in gathering ingredients for.
As they feed a Toadie a tracker and let it go, the Boogeyman is alerted and sets a trap for them. This leads Kelly and Liz to the very Halloween party Kelly intended to attend. A shadow monster awaits them in the basement, leading them into a series of misadventures and ultimately into a trap set by the Boogeyman himself. In a gripping sequence of events, Kelly narrowly escapes while Liz is captured. Simultaneously, Curtis brews the Angel Hair potion as Kelly locates and infiltrates a decrepit lighthouse where Jacob is imprisoned, with nightmares bubbling to the surface.
Against all odds, Kelly cleverly outsmarts the Toadies trapping her, and after regaining her friend Liz's senses, she confronts the Boogeyman in a climactic showdown. Armed with a monster punch, she delivers the Angel Hair potion to the villain, causing Jacob to awaken and dissipate the impending nightmare creatures.
Finally, after ensuring Jacob's safe return just before Ms. Zellman's arrival, Kelly is celebrated for her bravery and resourcefulness. Not only does she earn praise for successfully getting Jacob to sleep, but she's also invited back to babysit the following night. Liz assures her that she will recommend her for membership in the Order of Babysitters, while her crush, Victor Colleti, portrayed by Alessio Scalzotto, takes the opportunity to ask her out, promising a bright and adventurous future ahead.
