School teacher Frans van der Steg likes to make up stories. He tells his students he's expecting a mysterious letter that may lead to an exciting adventure. When he actually does receive a letter, he suspects the children are playing a trick on him.

Frans has been left at the seven way crossing at night. There he meets a young 'Brozem' who gives him a ride back to town. The next day, Frans goes out to investigate and discovers the mysterious letter was written by Count Grisenstijn.

On Sunday morning, Frans has a meeting with magician Thomtidom at his house 'Schijn en Wezen' ('Illusion and Existence'), who tells him the history of the so-called 'Trappenhuis' ('House of Stairs'), where Count Grisenstijn is supposed to live. But during the story Thomtidom serves the young teacher a spiked drink.

Roberto offers Frans some food at the Zevensprong. He asks the teacher to become a member of the conspiracy of Seven. When he arrives back home, Frans finds out his landlady is Roberto's aunt and another of the seven conspirators.

Frans and his students have a meeting with Miss Rosmarijn, the chairwoman of the conspirators. Together with Thomtidom she tells him about the prophesies of Count Gregorius Grisenstijn.

While searching for the pillar with the sign of a cat, Frans gets into a scuffle with Grisenstijn's men and is wounded. Roberto and coachman Toereloer bring him to Miss Rosmarijn to recover.

Roberto brings Frans to the 'Trappenhuis' ('House of Stairs'), instead of coachman Toereloer. But as soon as Roberto puts on his helmet, he becomes 'the Brozem'. And when Frans finally meets Count Grisenstijn, he immediately has a slip of the tongue.

Count Grisenstijn has hired Frans to be his nephew's teacher and invites him to stay the weekend. However, he will have to get there on his own accord, since coachman Toereloer has been fired. When he meets young Geert-Jan, the boy turns out to be quite a handful.

Geert-Jan has accepted Frans as one of the Seven, but pretends not to like him in front of his uncle. He tells him all about the prophesies and the treasure. Frans also meets the green-eyed Iwan. During his second visit, Frans and Geert-Jan find a secret doorway and a hidden path.

Each time Frans comes over to tutor Geert-Jan, they secretly spend some time searching the house. Frans' other students decide to write letters to the isolated Geert-Jan. Iwan the cat makes a discovery.

Geert-Jan's eleventh birthday is approaching and although he isn't allowed to invite any other children to his party, Frans can stay for the weekend. All the other conspirators as well as the schoolchildren prepare presents and letters for Geert-Jan.

Thomtidom secretly meets Geert-Jan at the crossing and gives him a special powder to sprinkle on Frans' pillow before he goes to sleep. The next day, Frans wakes up with green hair.

Frans has been fired because of his green hair, but Toereloer helps him get back to the 'Trappenhuis' ('House of Stairs'). Then all the children of his class arrive to celebrate Geert-Jan's birthday against the strict orders of his uncle.