Teacher Frans van der Steg tells the students in his class every day exciting stories, in which he himself plays the leading role as Frans the Red (because of his red hair). One day he can't think of anything and says he expects an important letter. To his own surprise, that evening he actually receives a mysterious letter sent to his home, signed by ?........?. The letter refers to a letter of application allegedly written by Frans, but he knows nothing of it. Frans decides to solve the riddle and his students are all supporting him. As was asked in the letter, Frans lets himself be picked up by a driver to go to the castle of the unknown sender. But on the way Frans gets into an argument with the driver. He steps out of the carriage in an open area in the forest: The Zevensprong. And there begins an exciting adventure full of mysterious events.