The first bell had long since sounded, but the corridors of High School No. 5 were full of the endless murmur of conversations, laughter and whispers. Nadia was standing with her nose in her phone, trying to finish the last sentence of her essay, when suddenly someone violently tugged at her sweatshirt sleeve. It was Simon, still with his sleep-deprived face and inseparable headphones around his neck. - 'Do you see this? - He asked, showing her the screen of his phone. A new network with a strange name flashed on the bright background of the school Wi-Fi app: "Look under the 2p stairs". - Is this some kind of joke? - Nadia furrowed her brow and glanced at Agatha, who had just joined them, slinging a heavy maths backpack over her shoulder. Agatha raised her gaze, her blue eyes sparkling with curiosity. - Maybe someone was hiding a clue? Or wants to contact us? - she chuckled. On that day, the third-graders had no lessons in the classrooms on the second floor. It was quiet and empty there, the light seeped through the frosted glass like through dirty glass. Together, half-jokingly and half-seriously, they headed for the staircase, which they usually steered clear of - mainly because of the legends of failed renovation attempts and a perpetually leaking roof. Under the steps lay a dusty, inconspicuous cardboard box. Nadia leaned over and carefully pulled out a small notebook with a crumpled cover from inside. Simon opened it at random. The first page bore a date from two years ago and some bizarre, seemingly random numbers and a name that neither of them recognised. - Who is "M.K.L.?" - Agatha asked, flicking through page after page. Some were blank, others written with equations, sketches and single sentences: "You'll find me at the bell" or "Time is running out". At the same moment, a metallic clang sounded somewhere on the second floor - as if someone had dropped a heavy object on the concrete floor. The three friends froze, exchanging quick glances. They had no idea what awaited them around the corner. Nadia squeezed her notebook tighter, Simon reflexively tightened his grip on the strap of his rucksack, and Agata was the first to move towards the sound, forgetting for a moment even the most important test of the week....