Leon and Maja loved puzzles and old buildings. When the old clock tower in their town was opened to the public, they couldn't pass up such an opportunity. Standing at the end of a cobbled street, the tower cast a shadow over the entire market square, and its clock face seemed to watch everyone who passed by. They started their tour with a winding staircase that creaked under every step. Old photos of watchmakers hung on the walls and the smell of old books and grease was in the air. At the very top, just below the huge clock mechanism, the guide - Ms Jadwiga - was waiting. She talked about the old secrets that the tower was said to have hidden, about lost tools and strange sounds heard at night. As the whole group admired the view from the window, Leon noticed a floor with carved initials. He bent down and touched the letter 'E' when suddenly his fingers encountered something hard. Sticking out of the hole was a small brass key! He directed a quick glance at Maja, who smiled in agreement. They both knew they had discovered something unusual. Instead of returning with the rest of the visitors, they waited until Mrs Jadwiga had closed the door and began to look carefully at the clock mechanism. They were looking for a place where the key could fit. At one point, Maja noticed a small door behind the gearbox, hidden behind cobwebs. The door was almost invisible, and just above it was the same 'E' symbol. Leon slipped the key into the lock and turned it slowly. A quiet click sounded inside and the door began to swing open. Beside them appeared a narrow corridor leading into darkness. A mysterious murmur came from the depths, as if something was waiting for them.... Would Leon and Maja go inside? What will they discover in the hidden corridor of the old tower? It all depends on their courage - and the imagination of the reader.