The illustration shows the Grool looking out from underneath the sink.
Katrina Merton and her younger brother named Daniel, have been allowed to skip a day of school to help their parents move into their new huge yellow house. While their parents busy themselves with unloading boxes, Daniel is put to work by feeding the family dog named Killer, and Katrina is told to wipe down the cabinets. While Katrina is cleaning the inside of the cabinet below the sink, she hears a noise from the far, dark corner of the cabinet. She ignores the noise and continues cleaning. Suddenly a hairy claw pokes out from the darkness and grabs her arm. Katrina screams and then realizes it is not a horrible monster, but just her little brother in his rat costume.
For some reason, the dog comes trotting into the kitchen. Daniel convinces her that he too is a giant rat and she falls for it. He calls her "Scaredy-Kat" and she responds by tickling him. While tickling her little brother, Killer the dog starts barking ferociously at the empty cabinet beneath the sink. The item to inspire Killer's wrath is revealed to be a sponge. When Kat goes to retrieve the contentious sponge, she discovers that the sponge appears to be breathing. Her brother at first is skeptical, but he still readily tries to claim the breathing sponge as his own, diving under the sink and hitting his head hard. He blames Kat, saying she pushed him, and the two try to justify their actions to their mother, who berates them for arguing over a "stupid sponge."
Katrina, armed with breathing sponge, goes to show the item to her father, who is on top of a ladder in their living room. Her father has nails in his mouth and he cheerfully agrees to examine Katrina's sponge, which by this point has sprouted two beady eyes. As he reaches down to grab the sponge, the ladder tips over and her father lands on the ground, grabbing his ankle. He angrily asks her, why she tipped over the ladder. Katrina insists she did not touch the ladder, as the sponge in her hand pulsates wildly. Katrina decides this is all too weird and disposes of the sponge in one of those big metal trash cans outside the garage.
A few days after moving in, Katrina is in the process of making a list of who to invite to her birthday party. She hears a racket upstairs and investigates. Katrina sees Daniel showing his friend Carlo the sponge she had thrown away. Katrina pleads with Daniel to re-throw away the creature. Carlo wants to spook his babysitter with the creature, and upon placing the "sponge" in a gerbil carrier, his hand appears to be bitten off.
After Carlo and Daniel have a good laugh about his prank, Carlo manage to get hurt. He does this, by stepping on a nail sticking out of a floorboard in Daniel's bedroom. Carlo lays on the floor in agony, as his sock soaks up the blood and the sponge in the gerbil cage pulsates wildly. Kat's mother goes over the details for her birthday party. Katrina is worried about topping last year's "Make your own pizza" party, which she claims was talked about for weeks after the fact. Katrina decides she is too old for themes. Her mother has promised to take her and her friends to Wonder Park. Katrina has only invited her friends and considers inviting her little brother, until he raises a big stink about her sponge, which she has now claimed full ownership of, and in fact is about to take to school to show her science teacher. She stores the sponge in a used potato salad container and heads off to school.
On the way to school a tree branch falls and nearly kills Katrina. Luckily Daniel pushes her out of the way just in time, spilling her bag out onto the ground. The sponge falls out the container and starts pulsating wildly.
At school, Katrina shows the sponge to her teacher. It just sits there like an ordinary kitchen sponge and the teacher tosses it back to Katrina like it was garbage. Then the teacher slams her fingers in her desk drawer, breaking both her hands.
Katrina walks back to class from the nurse's office, where she helped her teacher. Katrina knows she has got to find some kind of peace of mind, and she might have been searching everywhere were it not for Daniel running up to her in the hall. It seems he has found the sponge creature in a very convenient book called Encyclopedia of the Weird. It is called a Grool and is a mythic creature that causes and feeds off bad luck. According to the book, the only way a Grool can be passed on is if its owner dies. If the Grool's current owner tries to give it away, they will die within a single day. Daniel also shows Katrina a picture of a potato-like creature with teeth called a Lanx, which is the Grool's more dangerous cousin. A Lanx will attach to a person and drain them of all their energy.
Walking home, Katrina starts wondering if maybe the book could possibly have been right. Once home, Katrina's mother informs her and her brother that Killer has run away. Not to worry though, as Katrina's mother has called the police and they are out searching for the dog. Daniel and Carlo go out to look for the dog themselves. Katrina gets frustrated and blames the throbbing sponge. Furious, she throws the sponge across the room. As she looks down at her hand, she sees blood everywhere. She had slammed her hand down on an open pair of scissors which were resting on a desk. Clutching her bleeding palm, she walks over to the Grool, which has changed color from a dusty brown to a tomato-red color, and now appears to be laughing evilly.
The morning of Katrina's birthday, she gets dressed for her fun day at the WonderPark waterpark, except it is raining outside. Katrina naturally blames the Grool and pouts over her breakfast waffles, when her mother tells her that she cancelled her birthday celebrations. Katrina sulks upstairs and makes plans for taking care of the Grool that are so intricate that she gets out a notebook and puts pen to paper. The plan? To bury the Grool in the backyard when the rain lets out.
Katrina somehow manages to master this plan and after burying the Grool deep in the backyard, she can't find Daniel anywhere. Eventually she spots him cowering in the garage, afraid that something was going to happen to her for burying the Grool in the ground. Well, nothing happens to Katrina, but the backyard does not fare as well. The next day, all of the grass and flowers her father worked so hard to cultivate in the backyard have shriveled up and died, the entire lawn brown and dead. Kat concludes that the Grool is mocking and punishing her for burying it, and so she digs it up while she thinks of another plan.
The following day, Katrina's favorite aunt comes to visit. Katrina thinks she will know a lot about sponges. Katrina takes her aunt by the hand immediately upon her arrival and goes to show her the Grool, which has again reverted back to looking like a regular sponge. Her aunt throws the old dried up thing on the floor and leaves Katrina's room, laughing. After she is gone, the Grool immediately reverts back to its moist pulsating self. Kat gets frustrated and smashes the Grool into a million pieces with her textbook. But all of the pieces of the Grool pool together and reform as one entity.
Granted a reprieve from school when a teacher's conference is scheduled, Katrina relaxes at home, gradually completing an essay on why her family is important to her. She sneaks down to the kitchen for some milk and cookies and upon returning to her room she sees the Grool has disappeared. Frantically she tears up her room looking for the sponge. She finds Daniel and he tells her that Carlo just left and he must have stolen the creature. Fearful for her life, Katrina and Daniel put on their jackets and race out into the street, heading for the park where Carlo has likely gone to show off the sponge.
They do not get far before they spot Carlo splayed out in the street, where he has hurt his knee from falling from his bike. He limps home by himself as the two siblings scavenge the road for the Grool, which flew from Carlo's bike basket when he fell. Daniel thinks the creature landed in the sewer and so Katrina lifts a grate and sinks down into the sewer to investigate. After spotting a cadre of real rats swarming towards her, Katrina wisely decides to exit the sewer.
Carlo excitedly prances on his one good leg down to the siblings, cheerfully telling them that he has found the Grool. Katrina gives him a hug as Carlo presents her with a wet crumpled brown paper sack, which he thought was the Grool.
Carlo apologizes and tells Katrina that he broke his leg racing bikes with some older boys, and maybe they stole the Grool once it fell onto the ground. Katrina and Daniel ride their bikes over to the park, where sure enough, a circle of high school boys are leaning over the dried up sponge. Katrina tells the boys that it's her favorite sponge and asks to have it returned. The boys make fun of her and their ringleader, a tall blonde, tells her that it is their sponge now. Katrina, knowing how bad luck finds those in possession of the Grool, slowly walks away and waits for something bad to happen. It does not take long, as some children playing baseball lob a baseball right into the tall blonde's head. The boy drops to the ground and Katrina grabs for the sponge, while his friends rush to his aid.
Katrina hops on her bike and narrowly avoids getting hit by a large truck, which swerves at the last minute. Then not a few minutes later, her entire front tire shreds, sending her spilling into the street. She walks her wrecked bike home, listening to the croaking laughter of the Grool all the way. Steaming with anger, she starts pummeling the Grool, slamming it against the concrete with rage. Every horrible thing she spouts at the creature only makes it pulsate with pleasure, every hateful pummel feeding its joy. Daniel rushes over to her, pleading with her to stop, that it is just what the creature wants, for her to get mad at it, for her to feed it anger.
Katrina will not listen to Daniel and she runs a few houses down to their house. With Daniel and Carlo not far behind, Katrina enters the kitchen, shoves the Grool into the drain, and turns on the garbage disposal. The disposal makes a lurching sound and the Grool is propelled out of the drain, safely, and lands on the counter, laughing maniacally. Daniel again pleads with her to quit trying to kill it, that the Encyclopedia of the Weird specifically says that the Grool can't be killed by any means of force. That is when Katrina gets an idea.
She picks up the Grool and instead of inflicting pain on it, pets it. She then begins to coo sweet nothings into the Grool's ear, and softly rocks the creature with love. The Grool begins to lose its coloring and shrivels up. Katrina tells the Grool that she loves it and gives it a big kiss. The Grool shrinks to a small shrieking ball and then explodes into a million little particles. Katrina explains for her little brother that since the book said the creature couldn't be killed by means of force, she figured that it could be killed by means of love.
Except there's suddenly a scratching sound on the back door. It's Killer, the dog. Rushing outside to greet the dog, Katrina sees the dead grass and flowers reverting back to their previous state, filling with color as they come back to life. Killer starts barking at the kitchen counter again and Katrina finds a creature that looks like a potato, only it has razor-sharp teeth matching the Lanx's description.
Cover Art Gallery
- This book is mentioned in the eight Give Yourself Goosebumps book, The Curse of the Creeping Coffin.
- The title of both the book and television episode is a spoof of the classic film, It Came from Beneath the Sea.
- The title is mentioned in an episode of Happy Days titled "Tell-Tale Tart," which predates the book.
- As is standard with each first edition release of a Goosebumps book, the first edition would reveal the title of the forthcoming installment in the Goosebumps series, "Coming soon from R.L. Stine", on its back cover before its print the next month. The previous book, Monster Blood III, revealed the title of this (then unpublished) book to be "It Came from Under the Sink!" The final published title changes the word "Under" to "Beneath".