On Monday morning, Primary School 27 smelled of fresh paint, damp jackets and chalk, although the blackboards had long since been white and dry-erase. Someone had painted a huge solar system on the main staircase, and a new clock hung above the entrance to the old wing, which stubbornly stood at 12:07, although the principal swore it had been fixed on Friday. Lena stopped by the clock, cramming a box of sandwiches into her backpack. Next to her, Franek was hovering around, fiddling with the bracelet of his watch, which glowed blue, and Nela was running her finger over the list of announcements. At the very top was attached a new timetable, printed, in thick black letters. - 'I think this is a joke,' Nela remarked, squinting. - See: Monday, thirteen ten. 'Workshop 13B. There is no such thing. Lena leaned closer. Rows of letters rustled in the silence of the morning bustle: history, maths, Polish... and then this: "Workshop 13B - special classes". - Check on your phone,' muttered Franek, tapping his wristband. - Maybe someone had added it by accident. The schedule in the electronic diary also had 'Workshop 13B'. Invariably. Persistently. - How about the new classroom in the new wing? - prompted Lena and she glanced towards the bright corridor with the new door, which still smelled of varnish. - But why thirteen? After all, in our house the numbering ends at twelve. Nela shrugged her shoulders, but her eyes became attentive, as they always did when something didn't fit with the facts she had gathered. At the first lesson, Mrs Rose, their tutor, handed out their revised plans and told them that everything was going like clockwork today. Lena raised her hand. - 'Miss, and where is Workshop 13B? Mrs Rose blinked. - Where is what? - On the set. We have class there at thirteen ten. - Nela slid a piece of paper over the edge of the bench. - It must be a printer's error - the teacher smiled. - There is no such room. It's probably about the physics lab. I'll ask at the secretary's office. Before they had time to write down the topic, the bell rang in the corridor. A new one, with a tune that the parents' council had devised - a theme from some song that was supposed to make everyone feel better. But instead of the usual sequence, there was a single, long tone, metallic and cool. Then silence. The class stared at the door. Mrs Rose furrowed her brow. - 'They're probably trying something in the old wing,' she muttered. - 'Attention, we're working on. At the break, Lena, Franek and Nela set off towards the library. In the old wing, where the floor creaked and where even the hanging plants looked like they remembered the last century, everything was quiet. Quiet all too much so. The bell above the library door rocked slightly, although no one opened it. - 'Good morning,' Lena said, looking inside. - Did you... - she paused, as an old map of the school lay unfolded on Miss Eve's desk. A flake of glass in the display case reflected the morning light. - Ah, you mice - smiled Mrs Eve. - Are you looking for something specific? - Room 13B - fired up Franek. - We have such a workshop on the plan. Ms Ewa changed her expression and reached for her glasses. - Thirteen... B? - she repeated slowly, as if tasting the letters. She looked at the map, moved her finger across it, stopped at the rubbed-out legend. - Hm. In my version it ends at twelve. Anyway, the lower corridors are closed now, they're renovating. - The lower ones? - repeated Nela. - You mean the cellars? - Underground - corrected Ms Ewa in a tone as if she wanted to make it sound less scary. - The old acoustic studios, the instrument store, there used to be a choir rehearsal room. No one had been there for a long time. - She smiled encouragingly. - Better go to the common room before the next lesson. Lena was already about to nod when out of the corner of her eye she noticed something in the margin of the map. A thin pencil line, as if drawn by a child's hand, led from the library to... nothing. To a white spot under the stairs. Someone had written there in small letters: "0". And next to it, in pale ink, a barely visible note: "13B". - Can we... um... take a picture of the map? - she asked. - For the project. - For what project? - For... for geography,' interjected Franek quickly, until his glasses tilted. Mrs Eve looked at their faces; she could read them like book covers. She sighed, but nodded. When they left the library, the corridor seemed longer than usual. A drop-shaped light bulb dangled from the ceiling, blinking once in a while, as if keeping vigil, but also occasionally dozing off. The clock above the stairs still stubbornly stood at 12:07, although before, as planned, it was supposed to be the first break. - 'If it's a joke, someone's got it pretty good,' said Franek. His wristband vibrated. - 'Oi, look. It picked up a new net. "SP27-13B." Password hidden. - A Wi-Fi network with a room number that isn't there? - Nela raised an eyebrow. - 'Or it's where they're not looking for it,' Lena replied and looked towards the stairs to the basement. The chain on the handrail was tied with yellow tape with black letters 'Renovation'. But between the rungs was the shadow of a narrow corridor and a grey door. They returned to the classroom, but their heads were elsewhere. In history, Lena had written three things in her notebook instead of dates: "0 = under the stairs, 13B = underground, 12:07 = clock". Franek tried to hack into the network, but the wristband stubbornly displayed: "Access restricted. Authentication: sound". Nela, who had a prodigious memory, suddenly remembered that in the headmaster's office, in a photo from years ago, there was a large brass bell in the corner with a worn-out inscription. She couldn't remember which one, only that the letter B looked like an inverted three. At thirteen ten, when the bell should have played a cheerful tune, the whole school froze. Instead, somewhere deep, as if from underground, five quiet, clear tones flowed. As if someone was playing on crystal glasses. Franek looked at the wristband: 'Authentication accepted. Access: SP27-13B." - Let's go," hissed Lena. - 'There's a lesson after all,' reminded Nela, but she didn't sound convinced. Mrs Rose also looked like someone whose knees were softening. A group of teachers was gathering in the hall, the headmaster was talking into a walkie-talkie, and the caretaker, Mr Henio, was rubbing his hands in his apron and repeating: "It's just a rehearsal, just a rehearsal." As everyone looked towards the secretary's office, three fifth-graders flashed past the display case with the old medals and walked half a floor down to where the "Renovation" tape dangled languidly between the railings. Lena first slipped her head under it, then her hand, and finally squeezed her entire backpack through. Franek handed her his leather notebook with a sticker in the shape of a lightning bolt, Nela clutched the library plan under her arm. It was cooler downstairs. The light from upstairs was turning into plumes of dust, the corridor smelled of damp and old soap. The walls were the colour of creamy ice, and the matte tile floor reflected their silhouettes as if in a broken mirror. - Can you smell it? - whispered Franek. - Do you hear that... sound? Somewhere in front of them something was humming softly, as if a thin string was starting to sway from breathing. Lena reached out her hand and touched the wall. It was cool. To the left stretched a door with the numbers 01, 02, 03... 10... 12. By the last one, the number had disappeared. Someone had painted over it with paint. But up close, under the paint, you could see that there used to be two numbers and a letter. As if someone hastily wanted to hide something. - Try the torch - asked Nela. Franek clicked the armband. The beam of light cut through the dust and stopped on the outlines of the letters. Lena took a breath. - 'Thirteen... B,' she read in a whisper. The handleless, heavy metal door had a small window covered from the inside with a kind of frosted glass, like another era. A plaque, long since dusty, was screwed underneath them. They wiped it off with their sleeve. The inscription was simple, carved as if in stone: "ACOUSTIC STUDIO 13B". Someone had stuck an old note on the corner: "Only class 5c can enter". - After all, we are 5c," whispered Nela. Before they had time to digest this, a message flashed in Frank's wristband: "Lesson starts now". And then something rattled quietly behind the door, like metal against metal. A short buzzing sound came from the lock, too precise to be accidental. As if someone on the other side was typing in a code. - Maybe it was the director? Or Mr Henio? - asked Franek, but his voice sounded too loud in the empty corridor. Lena felt her stomach grow hollow and light at the same time, like just before a start at a competition. She leaned over to the window and put her eye to the frosted glass. She saw nothing, just a spill of light and... something that moved, like the shadow of a wheel. Nela put her ear to the door. For a moment, nothing came to them. And then - one thump, a second, a third, a fourth tone stretched like a rubber band, a fifth as short as a snap. Franek took a deeper breath. - 'If it's a special class, you probably need to...' - He looked at the panel next to the lock, to which a thin wire fed. The panel had a single slot and a note symbol. - Play - Lena finished. - Just like you did just now. - Five tones, right? - Nela made sure and tapped out the rhythm with her fingers against her knee. - 'I just can't remember which was the longest. Behind their backs something squeaked on the stairs. Someone was coming down, but very slowly, as if counting the steps. Lena felt the hairs on the back of her neck stand up. Franek raised his blindfold, the panel flicked on. There was a quiet crack in the silence, as if the first tooth had been unlocked in the lock. - Lena - whispered Nela. - Decide. The door vibrated, as if someone had laid a hand on it from the inside. A faint glow oozed from the frosted glass, which brightened in an instant, and then....