At the end of Bird Street, right next to a huge park, stands a school called Under the Oak Leaf. It is an unusual place - the walls are overgrown with ivy, and the oldest oak tree in the area grows in the middle of the school playing field. It is here that the adventures of four friends - Zosia, Filip, Nina and Tymon - begin. Zosia always wears pigtails and has hundreds of dragons drawn in her notebook. Filip knows all the trivia from all over the world (even about bats that sleep upside down in the school cellar in winter). Nina loves to read old books in the library and Tymon... Tymon just likes to laugh and invent things that shouldn't work but do. That morning, as it was slowly beginning to dawn and the air smelled of wet leaves, a strange message came into school: "The bell has gone missing!". The bell was no ordinary bell - instead of the usual buzzing, the sound of the breaks at the Oak Leaf School was announced by the widespread laughter of magpies, recorded by Mr Lark, the caretaker and inventor. Without this sound, the children did not know when recess was and when the lesson was. Everything suddenly stood on its head! The maths teacher forgot it was lunchtime and the librarian read stories to the children for two hours, disregarding the clock. The friends met under the oak tree during recess (or rather... what everyone thought was recess, because without the bell, no one was sure). "This must be some kind of mischief!" - stated Tymon, jumping up and down with excitement. "Or someone didn't want there to be breaks today," mused Nina. Philip took a miniature magnifying glass out of his pocket, which he always carried with him. "Maybe we can find some tracks?" - he suggested. Zosia looked at the others and made a decision: "We have to investigate! First, let's try to find out when the last time someone saw the bell." The friends decided to start in the basement, because that's where Mr Lark kept all his inventions and tools. The corridors under the school were full of creaking doors, old maps and mysterious switches. As they entered the cool cellar, a light flashed in the darkness.... Suddenly, from behind one of the doors, they heard a quiet, metallic clatter and a sort of... laughter that sounded nothing like a recording of magpies. The friends approached the door slowly, their hearts pounding like the bells on the school tower. Who or what was waiting for them on the other side?