At the turn of the 20th century, 10-year-old Azur lives in an orphanage and earns his living as a telegraph operator. Every day, he rides his bicycle and carries to their addressees the “petits bleus”, letters that come out of the pneumatic network under Paris. One day, as he was carrying one of these letters, he noticed that the mail was heavier than authorized. Inside, a coin. A gold coin. Azur is tempted: he keeps the coin and the bill he was supposed to deliver… The trouble and the adventure are just beginning.