Twelve-year-old Cole Johnson (Judah Lewis) faces relentless bullying from his neighbor, Jeremy. Fortunately, his babysitter, Bee (Samara Weaving), bravely stands up for him, scaring Jeremy away. When Cole’s parents decide to spend the night at a hotel, he enjoys a fun evening with Bee, but soon he receives a text from his neighbor and best friend, Melanie (Emily Alyn Lind), that piques his curiosity about what Bee is up to after his bedtime.
Expecting nothing unusual, Cole is stunned when he sneaks a peek and discovers Bee along with her high school friends: Max (Robbie Amell), John (Andrew Bachelor), Allison (Bella Thorne), Sonya (Hana Mae Lee), and Samuel (Doug Haley). They appear to be having fun playing a game of truth or dare, but the mood quickly darkens when Bee, after kissing Samuel on a dare, takes out two daggers and brutally stabs him in the skull. The shocking act reveals the group as members of a demonic cult.
In a panic, Cole dashes back to his room to call 911, slips on his shoes, and grabs a pocket knife for defense. He feigns sleep as Bee and her compatriots enter his room to gather a blood sample. Once they leave, however, he attempts to escape through the window but collapses from sheer exhaustion as Bee remains lurking nearby.
The cult interrogates Cole, trying to excuse their sinister activities as merely a "science project." When the police finally arrive, chaos ensues: Max kills an officer with a poker, while the other cop accidentally shoots Allison. As Cole desperately tries to outsmart the captors, he takes off fleeing upstairs, where he unwittingly pushes John over the banister, resulting in a fatal impalement.
Cole's escape leads him to hide in the crawlspace beneath his house. Even though Sonya discovers his hiding place, he manages to trap her in the basement before he concocts a plan using a firework rocket and bug spray to eliminate her. After witnessing this display of creativity and resourcefulness, Max pursues him but meets his own demise in a tragic fall.
Seeking refuge at Melanie's house, Cole realizes he must ensure her safety, expressing his apology for dragging her into his harrowing ordeal. Despite his assurances, Bee is hot on his trail, and back at his own house, Cole confronts Allison, who lunges at him with a knife. However, she is abruptly shot in the head by Bee, who reveals that she had once made a pact with The Devil, trading innocent lives for power by sacrificing them and reciting verses from an ancient book.
While Bee attempts to coax Cole into joining her dark cause, he fiercely refuses and bravely incinerates her spellbook. Now determined, Cole races to Melanie's house to commandeer her father's car and heads straight back home as Bee tends to her book in the living room. In a climactic moment, he crashes into her, resulting in a heart-wrenching goodbye before he climbs from the wreckage.
As sirens wail and emergency responders arrive, Cole confidently informs his parents that he no longer needs a babysitter. However, the nightmare isn't over yet, as a firefighter, while investigating the house, becomes the next unwitting target of Bee's wrath.
