Every time Maja clamped her mouth over the metal wires of the braces, she repeated to herself that this was a long-term project: two years of discipline in exchange for a smile that would not shy away from any photo. The phone prompted a timer: two minutes in the morning, two in the evening, floss once a day, follow-up appointments every six months. So much theory. Practice had a mint taste and sometimes hurt. That evening the mint hung in the air thicker than usual. The school turned into a maze of lights and whispers, as Science Night was underway. Microscopes glittered in the biology room, test tubes were rearranged in the chemistry corridor and arrows were attached in the basement, in the old tiled wing: Smile Club - discover the tile! - Who came up with that name? - snorted Kacper, correcting his baseball cap. - Like an advertisement for baby paste. - 'Mrs Roma,' replied Ula, who knew all about all teachers. - But it works. Look how many people are going. And besides... - She looked at Maja and winked at her. - Your lot. Maja touched the zipper of her camera with her tongue, as if to make sure everything was sitting in place. Her backpack flopped against her hip, heavy with the water and flushing fluid she carried with her out of habit. They walked down the stairs, where the tiles were cool and the echoing footsteps carried. The closer they got to the door marked "Cabinet 13 - closed", the stronger the smell of mint became, as if someone had just walked out and left a cloud of rinse breath behind. Folding tables were set up in the study. On one stood a UV lamp, on another the staining tablets, the ones from which the tongue turns purple. Mrs Roma, in a white smock, smiled broadly. - Welcome to the show! Let's see what plaque really looks like. No scaring, but the truth - she winked. - Who would like to come? Kacper raised his hand to make it funnier. Ula took up the challenge. Maja took the tablet without hesitation. As she chewed it and spread it with her tongue, she felt a selective tartness. In the mirror, her teeth were fine at the incisors, but there were bluish purple streaks at the molars. - Just look! - Kacper laughed as his tongue looked like a cartoon and his teeth had dark dots on contact. - Okay, the thread is not a stitch. - I said - Ula showed her results, minimal, almost clean. - Mum always says: "pick up food scraps with the floss, not the nail". - And now UV - Mrs Roma announced, dimming the light and shining the lamp on their smiles. In purple, the spots shone like stars on a map of the sky. - See? Where it's harder to reach with a toothbrush.... Maja bit the corner of her lip, then laughed involuntarily. It was strangely exciting - to see the truth and not just feel it on her tongue. She also felt it was a good time to stop postponing flossing until tomorrow. Then a quiet sound came from the depths of the corridor. Like the hum of a small turbine, which once intensified, once quieted. For a second everyone was silent, listening in. Mrs Roma tilted her head. - Probably the ventilation. 'Everything is old in this part of the school,' she said, but Maja noticed how carefully she put the lamp down. Someone opened the window. The current of air brought a stronger smell of mint, cool to the point of shivering. Maja glanced at the door "Cabinet 13 - closed". The lock had been forecasting calm for years, but now a drop of water glistened on the handle. And... something else. In the UV light, tiny, smudged marks appeared on the tiles just below the door. As if someone had dropped a few drops of dishwashing liquid and walked over, leaving a streak of phosphorescent blue. - Do you see that? - whispered Ula. Mrs Roma wiped the doorknob with her handkerchief, as if to check whether the stain was fresh. The phone rang in her pocket. She looked at the display and groaned. - Director. I've got to take this. You guys stay here, don't touch anything. I'll be right back,' she chuckled and walked out into the corridor, putting the camera to her ear. A semi-silence fell, interrupted only by the teacher's footsteps, which fell silent around the bend, and that noise - as if something was turning, but too far away to name. Kacper put down the phone with which he had been recording everything up to that point. - It's... a bit cinematic,' he admitted. - But seriously, where does the mint come from? And why is UV catching traces? - A rinse with oils and dye. Some of it has fluorescence,' whispered Maja. - Also, dye tablets too. But who would spill it here? Ula knelt down, examining the dots. - Someone who was in the dark. Or didn't care about the mess. Before they could think of anything cleverer, the lock on Cabinet 13's door clicked. Not loudly, but distinctly, as if metal was coming into contact with metal after years of quiet. The door swung open by a centimetre. Cool air blew in their faces. - 'Let's not do anything stupid,' Maja began, but at the same time she leaned over to look into the crack. There was semi-darkness inside. She saw the outlines of a dentist's chair, covered with a ragged sheet. A faded poster hung on the wall: "Floss - your gums will thank you", and next to it a model of a giant tooth, on which they had once probably demonstrated how to brush "from gum to tooth". Someone has left a paper cup of water on the countertop. The mirror above the washbasin was fogged up, even though the office was cool. On the steam, someone had drawn a crescent moon with their finger. - 'OK, I can't stand it,' Kacper pushed the door a little wider. - 'Let's just see if there's anyone in there who needs help. Or... I don't know. Maja felt the familiar pressure between her shoulder blades: the one that always appeared when she didn't want to be afraid, but was afraid nonetheless. She crossed the threshold first, because she didn't want Kacper to be the one to get into something pointless. Ula came in right behind her, holding the UV lamp to her chest like a torch. The floor creaked, but the study was clean. Too clean. Not like the abandoned rooms, with finger dust and cobwebs in the corners. Here, the dust had settled only in places no one had touched. Metal tool trays gleamed. On one lay fresh dental floss, unrolled for several centimetres, as if someone had interrupted it in the middle and walked away. - Who is allowed in here anyway? - Ula said in a whisper. - This surgery had been closed for years. - Maybe someone opened it for Science Night and forgot to close it? - suggested Kacper, but it sounded weak even in his mouth. Maja walked over to the mirror. The foggy surface trembled like milky water. She wiped the corner with her finger. In the misty light she saw her smile with thin dark blue shadows by the camera locks, something the tablet had not yet managed to wash away. She leaned in more and saw that someone had written on a pair of letters that had almost disappeared: "Remember your tongue". - About the language? - she repeated in a half-hearted voice. - 'Well, brushing your tongue removes bacteria and your breath is fresher,' Ula reminded automatically. - But who... Suddenly there was a soft, steady sound, as if someone had pressed a pedal by the armchair. The light at the artist's desk above the armchair flickered, sizzled and glowed in a pale circle. In the corner of the study something murmured. It wasn't a fan. It sounded like an old micro-motor that doesn't get full power, but still tries to move. - 'After all, there's no electricity in here,' whispered Kacper, taking a step back. - There is - Maja looked at the power strip on the wall. The LED was glowing green. - Someone had connected the bypass. Look. The murmur became clearer. The pulsating, rhythmic breathing of a polishing machine began to come from behind the curtain that separated the study area from the small back room. Ula lifted the UV lamp and moved it across the tiles. In the violet light they saw something that made them hold their breath: on the floor, from the threshold to the back room, someone had drawn an irregular line with long strokes of toothpaste. The line meandered like a river and ended just at the crevice of a pale pink curtain. The toothpaste was fresh, shiny and its scent pierced through to tears. - This... - Kacper didn't finish. - Someone leading us? - Or warning us,' Maja said, although she herself didn't know where the idea had come from. On a metal tray next to the chair vibrated a small jar labelled 'Biofilm indicator - overnight'. Inside, the liquid moved as if on its own as an invisible machine growled in the study. Maja reached out, but held back at the last moment. Her fingers trembled. She didn't like dirty surprises near her teeth, and this felt just like that: like foam that someone had spread across the floor to leave a trail. - Hear that? - Ula lowered her voice to a barely audible whisper. Now everyone could hear. Apart from the monotonous muttering, there was something else: a rhythmic tapping, as if drops were falling against a metal bowl. They stared at the spittoon beside the chair. Its porcelain bowl glistened and water swirled inside. Indeed - someone seemed to have just rinsed out their mouth and pressed the lever. The water flowed, although no one touched it. - This is not funny - muttered Kacper, but his eyes lit up with curiosity. Maja took another step. The curtain moved slightly, as if someone on the other side was stretching their fingers. She reached out and grasped the edges of the fabric. The mint pinched her nose and her heart rumbled in a rhythm that merged with the vibrating sound of the device. Just above their heads, the lamp hissed and dimmed. On the wall, in a frame, hung a smiling poster: "Don't be afraid of the thread. She will save you." - On three," she whispered. - One... two... As she pulled the curtain, the air became icy and a shape emerged from the darkness....