Frances Bacon McCausland, an intelligent and level-headed girl, starts high school a year early in the small town of Middleberg. The town has been buzzing with strange occurrences: dogs appearing on roofs, alarm clocks ringing hours early, eggs splattered on a teacher’s car, gelatin in the school swimming pool, and the letter “B” sprayed across town. The Bs also appear on the school lockers—except Frances’, which bears a hidden B on the inside. All of these pranks seem to point at Frances, though she can’t explain who is behind them or why.
Larry Houdini, an older boy who claims to be an imaginary friend visible only to children, steps in to help. He tells Frances that she is being framed by the Boogeyman, and she struggles to accept this explanation. The town’s tension escalates when the Boogeyman triggers a blackout, a gesture foreshadowed by the sprayed Bs; yet the McCausland home remains lit, its Christmas lights shining as if untouched by darkness. In the midst of the chaos, Frances’s friendship with her best friend Joanne Smith begins to fracture as she tries to convince others that Larry exists, while her family starts to doubt her sanity.
To understand what’s happening, Frances checks out The Boogey Book from the library for Larry. Inspired by its pages, Larry designs a contraption called a temptrifuge that could age the Boogeyman into a harmless, old man. Along the way, Frances uncovers a surprising truth: Larry is not merely a helpful apparition—he is connected to her brother Darwin’s past, because he was Darwin’s imaginary friend too. Frances eventually convinces Darwin to grow up and move beyond belief in his imaginary companion, which complicates the dynamic between Frances, Darwin, and Larry.
Unsettling discoveries pile up when Larry concocts Boogey Goo as bait, and it proves delicious to him in a disturbing twist. As Frances digs deeper into the book’s origins, she learns that a Boogeyman is born when the creator of an imaginary friend stops believing too soon. Then tragedy strikes: after accidentally stepping in Boogey Goo, Darwin attracts the Boogeyman and is kidnapped while sitting in Frances’s room. Frances and Larry race after him into the Boogeyworld, a shadowy realm that lies beneath Frances’s bed.
In the Boogeyworld, the stakes rise as Larry temporarily becomes a Boogeyman himself—transformed by Darwin’s lack of faith in him—while another Boogeyman drags Darwin toward a dangerous cliff. Frances must find a way to rekindle belief, and she does: she convinces Darwin to believe in Larry again, restoring him to his human form. With the temptrifuge in hand, Frances confronts the Boogeyman and discovers that its true form is Zoe, Frances’s long-lost imaginary friend she stopped believing in when Darwin fell ill.
Frances realizes that her decision to grow up and let go of certain beliefs created a vulnerability that allowed Zoe to fade. Yet she still cares and reaches out, taking Zoe’s hand to help her return to normal. After freeing Darwin, the siblings return to the real world, where their parents reveal that similar mischief is unfolding in another city. Larry reveals that the “guy in his head” has orders to deal with the other Boogeyman, and Zoe volunteers to help—though she is still learning the ropes of being a Boogeyman.
The departure is bittersweet. Frances is still unsettled by how hard it is to believe in these beings again, but Larry reassures her that childhood memories can coexist with adulthood as long as she holds onto a sense of wonder. He shares a final kiss with Frances, then switches on the Christmas lights outside, allowing him and Zoe to depart together. That night, Darwin, frightened, is sent to Frances by Larry, and she invites him to sleep beside her. As the lights glow softly, Larry and Zoe watch over them with knowing smiles, suggesting that belief, once stirred, can endure even when growing up continues to pull at the edges of wonder.
