The school library had always seemed like a place full of secrets to Amelia, but on this day it all felt different. Together with her friends - Max, the computer expert, and Igor, who never parted with his camera - she was looking for books for a history paper. At the end of the furthest bookcase, behind some dusty volumes, their attention was drawn to a huge old clock with a carved face. "Have you ever seen anything like this?" - Max leaned over, looking at the hands, which moved in a strange way, sometimes even backwards. Igor, as usual, started taking pictures. Amelia noticed that old parchments and strange metal parts resembling gears were stacked around the clock. When Max, encouraged by curiosity, touched the brass crown of the clock, the whole mechanism suddenly trembled. The hands began to spin faster and faster, and a mist appeared around them, drawing all three of them into the centre of the dial. When the mist cleared, the children found themselves in a place completely unfamiliar to them. Instead of shelves of books, they were surrounded by cobbled streets and horse-drawn carts. People wore clothes as if from a bygone era, and somewhere in the distance the clatter of a forge echoed. Amelia tried to find any landmarks, but everything seemed alien and fascinating at the same time. Max nervously looked through his phone - this one, however, refused to obey. Igor was amazed to see that the camera only showed blurred images. Suddenly, a boy wearing a felt hat and with a mysterious smile approached them. Without a word, he handed Amelia a small silver key. "If you want to go back, you must find the Heart of the Clock," he whispered, then disappeared into the crowd. The three friends looked at each other with a mixture of fear and excitement. What did this puzzling clue mean? Where should they start looking? And would they even be able to return to their library? At that exact moment, there was a strange metallic sound and something very unusual began to happen around them....