Deep in the woods, in a crooked house full of stairs, a young boy is kept prisoner by his uncle. He cannot meet other children, or have any friends. He holds the key to a secret. Meanwhile, in a quiet village, Frans the schoolteacher invents incredible stories of perilous deeds, shipwrecks, desert islands, and haunted castles to entertain his pupils, in which he is the hero. Then one stormy evening, a mysterious letter blows onto his doormat, summoning him to a meeting. Suddenly, Frans is on a real-life mission, one in which he will encounter magicians, secret passages, conspiracies, hidden treasure, a black cat with green eyes and a sealed parchment which predicts the future.