A mysterious chalk star. A strange code. And three young detectives who can't resist a puzzle.

When Leo discovers a carefully drawn star on the school playground with the numbers 3–1–4 beside it, he knows it is no ordinary piece of chalk. The symbol belongs to the Tree Detectives—and someone seems to know exactly how to get their attention.

Together with Mira and Sam, Leo begins following a trail of secret messages, hidden clues, codes and ciphers. A book code leads them to the school library. Chalk dust points them through quiet corridors. Mysterious symbols, concealed notes and puzzling instructions draw the three friends deeper into a mystery that seems to have been created especially for them.

But who is leaving the clues—and why?