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Book 1: Mr Summerling's Secret Code
Marley's best friend has moved to Chicago. The girl misses her very much, writes letters to her friend and waits for a new meeting. But instead of a reply, she receives news of a meeting with a lawyer. Marley's classmates, Isla and Cy, also come to the meeting. It seems that Mr Summerling, an old neighbour, has left the three of them a treasure that they have to find using the clues he left behind. Could he have left the children money? Then Marley could have bought a plane ticket to Chicago. But the adventure gives the girl something much more than money - new friends."

Book 2. The Secret Room
A few days later, the children find new riddles and clues left by their elderly neighbour in a hiding place under the floor of the tree house. Mr Summerling invites Marley, Cy and Isla to his house, full of secret rooms, secret staircases and ghosts. But there is a time limit to complete this quest. Will the children solve the neighbour's new, much more complicated and frightening riddle, since they will always be under the watchful eye of their enemy?