During a midnight school game, Karolina, Kacper, and Matthew follow toothbrush clues into a hidden tunnel beneath the biology room and discover an underground world where something in the dark is waiting for them.
It was just past midnight when Professor Zgryza High School stood in the torchlight like a row of wet teeth, gleaming and uneasy against the dark. Karolina, Kacper, and Matthew moved along the corridor on silent feet, trying and failing to keep their laughter down. What had started as a school anniversary game - a scavenger hunt built around the "I take care of my teeth" campaign - felt more ridiculous than scary. Toothbrushes hung from door handles. Tape arrows pointed toward toothpaste posters. The teachers had gone all in.
"This is so dumb," Kacper whispered, grinning as he nudged a cardboard molar taped to the wall.
Karolina smirked, but the sound died in her throat when they reached the biology room. The door was unlocked. The room was empty. Their beams swept over the desks, the sink, the model skeleton in the corner - and then stopped under the blackboard.
A strip of parquet had shifted out of place.
Matthew was the first to kneel. He ran his fingers over the edge of the loose board, then pressed harder. The plank gave with a dry crack. Cold air breathed up from below, carrying the smell of damp earth and something sharp underneath it. Mint. Not toothpaste exactly. Something older.
Kacper swallowed. "Tell me you smell that too."
Before Karolina could answer, Matthew pried the board farther open. A black gap yawned beneath the floor, narrow at first and then deep enough to hide all three of them. No one spoke for a second. Then Karolina crouched and saw a metal rung disappearing into the dark.
"We should not be doing this," she said, though her hand was already on the edge of the opening.
"Which means we absolutely are," Matthew muttered.
He dropped down first. Kacper followed with a muttered curse, and Karolina went last, the room above sealing into silence as soon as her feet left the floor. The tunnel was too tight to stand in. Brick scraped their shoulders. Moisture beaded on the walls. The mint smell grew stronger with every step, like someone had just brushed their teeth somewhere far below.
At the end of the passage, the space widened into a cave.
Karolina stopped so fast that Kacper bumped into her back. The walls were covered in a mosaic of a giant jaw, every tooth tiled in pale stone. Overhead hung a chandelier made from loops of dental floss, its threads catching the faint green light that seeped from somewhere deeper inside. Beneath the mosaic, hidden between the teeth, were three buttons: a toothbrush, a coil of floss, and a tiny tube of toothpaste.
Matthew reached toward the toothbrush first.
The lights cut out.
A low hum filled the cave, vibrating through the floor and into their bones. Kacper fumbled for his phone, but the screen stayed black. Matthew swore under his breath. The green light flared brighter at the far end of the chamber, and a shape began to separate itself from the dark - tall, thin, almost skeletal, with a smile so bright it seemed to reflect from the walls. Silver braces flashed at the corners of the mouth.
"Only those who know the true power of oral hygiene will pass," a voice said from the shadows. "The rest stay here."
Karolina could not see where the voice came from. She could only hear the slow, deliberate footsteps moving closer from the tunnel behind the figure, and the thin, terrible click of metal against stone.