Adventure School No. 12 was ordinary, if you didn't count the fact that the class 8B teacher was called Clementine and kept dried mangoes in her drawer instead of pens. It was at this school, in a dingy, dusty basement, that the Invisible Hamsters Club was founded. The founders of the club were Maks (known for his crazy ideas and lack of sense of distance - especially to the walls), Zuza (a master of irony and a fan of all things cute) and Igor (shy, but with a talent for drawing cartoons so realistic that once even the headmaster took a drawn "Do not disturb" sign as authentic). The basement was the perfect hiding place: there were bicycles forgotten by everyone, plastic sleds from a previous era and a poster depicting a sheep with an electric guitar. The three friends set up their club in an old wardrobe that still remembered the days when the school cook had hair on his head. They equipped it with cushions, batteries of chocolates and an ingenious plan - meet every Friday to come up with "something really stupid that won't get the police". On the first Friday, they decided to have a hands-free jelly bear eating competition (Zuza won, Maks swallowed a dragon as a consolation prize), on the second Friday, they tried how long they could pretend they were a Bermuda triangle (for almost an hour, until the caretaker spoiled the fun by demanding they hand over their broom). But on the third Friday, something changed. Maks was the first to arrive in the basement and noticed that the club's cupboard was slightly ajar, although the previous day they had clearly locked it with a makeshift padlock made from a hair elastic. Inside, their cushions were scattered about and two bars of chocolate had disappeared from their supplies. On the floor lay Igor's tangled notebook with a drawn figure of a hamster in a superhero cape. Maks rubbed his eyes. Was someone watching them? When Zuza and Igor arrived, Maks was already whispering excitedly, showing them the mess. Zuza thought that perhaps the school had acquired a new breed of mouse - chocolate-nut fans. Igor, however, immediately noticed something strange: one word had been written on the edge of the hamster card in crayon. "Get ready." The three friends stared at the notepad and then at each other. For a moment there was perfect silence in the basement, broken only by the distant clatter of the school pipes. Then a deafening rustle came to them from behind a cupboard, as if something small and very fast was sneaking across the dark floor....