As Ola crossed the threshold of High School No. 7 that morning, she felt a familiar tingling in her stomach. Something hung in the air - the crowd of students was uncharacteristically already swirling from the entrance. A large yellow banner hung next to the portrait of the first principal: "ATTENTION! RENOVATION! SORRY FOR THE INCONVENIENCE". Nothing unusual, but the school had looked exactly the same for years. Always the same colour of the walls, always the same creaky floor and iron bookcases in the library. Ola quickly found her friends Kacper and Maja. Kacper, tallest in the class and perpetually dishevelled, was already pulling out his phone to take a picture of the colourful barriers standing in the corridor. Maja, on the other hand, with notepad in hand, was writing down everything said by the janitor circulating since morning and warning against going up to the second floor. - Have you seen what's going on upstairs? - Maja asked with a glint in her eye. - Apparently the workmen have found something up there. The whole floor is locked up! Even Mr Adam from chemistry couldn't get into his office. - Maybe they will finally fix the leaking roof - Kacper muttered, glancing at the builders removing the old planks. The classes passed quickly. After lessons, the three of them looked at the warning tapes in the corridor separating the stairs leading to the forbidden floor. Finally, Maja couldn't stand it: - Come on, we'll try to get up there. What kind of pack are we if we don't take advantage of such an opportunity? Ola looked around. Most of the students had already left, the corridor became quiet. She threw her backpack over her shoulder and set off after her friends. They climbed the creaking stairs, listening for the sounds of the workers. Fortunately, they were all hanging around at the other end of the floor, and the old corridor leading to the locked room 24 seemed quite empty. Suddenly Ola noticed something under the windowsill. A small metal object. She bent down and picked up a rusting key that someone had probably lost in their haste. Kacper ran up first. - 'Hey, maybe it's from one of those locked rooms? - he chuckled with a broad smile. Maja was already holding half-open a heavy wooden cabinet at the end of the corridor. Through the glass in the door, something shiny could be seen, as if a metal rod or a piece of old machinery was hidden there. A strange light was shining in the middle of the hall - but after all, all the lights should be off! Before Ola had time to say anything, they heard someone's footsteps behind them. Someone was approaching them quickly from the direction of the stairs, and the key began to slip from her hand....