Ignaz loved to play in his room, where there was always something interesting going on. One morning, when the sun was shining through the window, Ignaz noticed that his rug by the door was a little crooked. When he corrected it, a small silver key with a blue pebble slid out from under it. - Hania! Come quickly! - he called out to his older sister. Hania ran into the room and stopped in surprise. They both looked at the key from all sides. - Maybe it's the key to the treasure chest? - whispered Hania with sparkling eyes. - Or to a door we've never seen before! - added Ignaz, letting his imagination run wild. The children started to look all over the house for a lock that would fit the secret key. They tried to open the letterbox, the old cupboard in the living room and even the toy box with it. However, the key did not fit anywhere. Finally, they sat down tiredly on the hallway floor. And then Igor saw a thin line on the wall behind a large plant in a pot. - Look, Hania! - he whispered. Behind the plant was a small, almost invisible door, which they had never seen before! Ignaz put the key in the lock and.... the door began to open slowly, creaking quietly. The children looked at each other with wide-open eyes, ready to discover what was hidden behind the door....