Lucy "Lucille" Dark is spending her summer vacation scaring her younger brother and participating in the local library's reading program. She scares her brother, Randy "Randall" Dark, by merely mentioning that there might be a monster behind a tree, in a bush, around the corner, etc. Randy falls for this because he is six years old. The reading program at the library is called Reading Rangers and the deputy of this program is the hideous, bald, sweaty mole-like man named Mr. Mortman.
Mr. Mortman tries to encourage Lucy to read the classics, but the only book she really digs is Mary Shelley's Frankenstein. She asks whether White Fang is about a monster and she also asks if Anne of Green Gables has any monsters in it.
One rainy summer evening, Lucy leaves the library and realizes she forgot her roller blades. She goes back to the library after it has closed and while hiding sees Mr. Mortman reaching into his desk to procure some flies to feed to his pet turtles, which he keeps in a metal pan on his desk at all times. Lucy sees Mr. Mortman's head swell up, his eyes bulge out, and his mouth turn into a gaping black maw. Lucy leaves in a hurry and forgets her roller blades again and so she heads back to get her roller blades, but then the door is locked.
She tries to tell everyone who will listen about how Mr. Mortman is a monster. But they do not listen to her because she is always talking about monsters. So a week later, she returns to the library and after Mr. Mortman gives her a new book for the reading club, she hides in the stacks to catch Mr. Mortman in the act. After he escorts everyone out of the building, Mr. Mortman locks the door.
Lucy says to herself, "This is boring." Then Mr. Mortman turns into a monster again! Then he eats some flies again. Then he picks up a turtle, chews up the shell, and swallows it.
Lucy runs to the locked door and realizes the door is locked from the inside. She makes it out of the library again. She runs home and sees her friend named Aaron, who she then tries to convince that she saw a monster. He does not believe her at all. She goes into her house and tries to convince Randy about Mr. Mortman, but he also does not believe her.
There is a knock on the door. It is Mr. Mortman. He kindly returns Lucy's bookbag, which she had left in the library. He asks if she had stayed behind after he closed up, but she says no. He bids her farewell, as her mother pulls up in the driveway.
Her parents will not listen to her, so she enlists Aaron's help. Lucy offers him five dollars in advance to accompany her to the library. Her newest plan involves her and Aaron going back to the library. They hide, he sees Mr. Mortman turn into a monster, and then they plan to both tell her parents. Unfortunately, on the day of the big plan execution, Aaron is nowhere to be found and seen. So Lucy decides she will go back to the library again.
Lucy again hides in the library, alone, waiting for Mr. Mortman to change into a monster. Except this time, she has brought a camera to snap his picture. Once Mr. Mortman transforms into a monster again, Lucy takes his picture, but forgets about the flash and Mr. Mortman flies into a rage. There is a chase scene and Lucy escapes the library one more time. That night, Lucy's family all goes to the mall to eat at the Chinese restaurant. Lucy pleads with her father to get her roll of film developed so she can prove to him once and for all that Mr. Mortman is a monster. After dinner, Lucy races to the one hour photo joint to pick up her picture of Mr. Mortman. Except the photograph shows an empty desk in the library, Mr. Mortman did not show up in the picture at all.
Never deterred, Lucy has to come up with a new plan. It still involves hiding and spying, this time her and Aaron are scouting out Mr. Mortman's home rather than the library. The two children tail Mr. Mortman from the library to his house. Lucy peeks through the living room window, just in time to see Mr. Mortman indulge in a series of grotesque meals. First he takes a big handful of fish from his aquarium and shoves them into his mouth. Then Mr. Mortman chews up an entire snail, shell, and all. Following that, he slurps up an entire eel-like a spaghetti strand. Lucy tries to get Aaron to come peek in the window, but he is off looking for a ladder to see into Mr. Mortman's house better.
Lucy loses her balance and falls from the window. Aaron runs away from the house, as Mr. Mortman exits the house and picks her up off the ground. Lucy makes up a story about how she was just there to tell him she would not be at the Reading Rangers meeting the next day. He offers to let her inside so she can call her parents for a ride home, but she does not want to go inside.
Lucy's parents will not let her quit Reading Rangers. Forced to attend her meeting with Mr. Mortman, Lucy braves the rain and returns to the library. Before he starts the meeting, Mr. Mortman gets up and locks the door. He tells her he can't let her leave. He starts to make his move towards her when she leaps up and attempts to escape. It seems dire but then she comes up with the bright idea of tearing out a drawer of the card catalog, spilling the contents on the ground. Mr. Mortman, though still a monster, is also a librarian and he stops his pursuit of Lucy to organize the cards. Lucy runs out into freedom. She also runs out into Aaron, who had been hiding in the library. Turns out he saw Mr. Mortman turn into a monster and is willing to tell her parents.
Lucy and Aaron tell Lucy's parents about Mr. Mortman and they respond with, "Well, I guess we have to invite him to dinner." A few nights later, Mr. Mortman shows up for dinner. The family makes polite small talk with the librarian, who eventually inquires as to what is for dinner and asks the father to repeat himself. The father says "You are!" again and he starts getting scared then Lucy's parents, who are monsters themselves. Lucy's parents then eat Mr. Mortman alive (off-screen), as their children cheer with joy, which they turn out to be monsters themselves.
Cover Art Gallery
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- This book and television episode title, along with some of the story itself, spoof the classic children's fable, The Boy Who Cried Wolf.
- The cover illustration features a girl looking in the dark library room toward a man looking like Danny DeVito with glasses and a cap holding a fly.
- The reprint is VERY bare-bones. Just the story and the "About the Author" page. No previews and no advertisements, not even an order form.
- Lucy and Randy's father was mentioned by name: Larry Dark.
- This was the first Goosebumps book to reveal that the main hero isn't really human after all.