Billie Turf and Dolfie have to stay at boarding school during the summer holidays because their report cards are too poor. Everything goes smoothly until Kwel comes up with an invention that turns water into gasoline with a pill.

Billie Turf is a student at Dennenheuvel boarding school. According to his teacher Kwel, he is unreliable, a good-for-nothing, and a liar who can only think about good food. Due to his boorish behavior during various activities, such as training for sports days, washing a car, and going on a fishing trip with his teacher, Billie repeatedly comes into conflict with the boarding school's management, which results in him receiving the same punishment every time: cleaning duty and going to bed without dinner. He is only resourceful and clever when it comes to obtaining food. His good nature is finally revealed when, together with Jossie, the director's niece, he manages to find the kidnapped student Kareltje and the ransom money.

Sjors and Sjimmie have received a magic ring as a thank you for their help to a well-known archaeologist. They end up in the Middle Ages with their regular opponents, Boris and Pedro, because of the magic ring. The magic ring makes wishes come true, but Sjors and Sjimmie don't know that. They are accused of witchcraft and are in danger of being burned at the stake.

Colonel Brulgraag has found the will of King Krijn XXIV of Schiermeeuwen. Whoever finds Krijn's Sword will be the new king. The colonel asks Sjors and Sjimmie to search for the sword so that it can be taken to a museum and not used by Krispijn to gain power.

Cor and Jan are two Dutch students who work in a hospital. A smallpox epidemic has broken out, but the country's capital is occupied by the army. Nevertheless, someone has to go outside the city to get a serum. Cor and Jan steal an old car and make their way through the military lines. It's not easy, but they're smart guys.

Uncle Theodor has died and is giving three million guilders to whoever finds the blue diamond. There are two heirs, Mrs. Kakel and Mrs. Ter Beek. They have seven days to find the diamond. Both receive a tape with information on how and where to find the diamond. Mrs. Ter Beek goes searching with her son. Mrs. Kakel, however, has Bram, Lam, and her son follow Mrs. Ter Beek so that the three of them can take the diamond from Mrs. Ter Beek. Who will have the last laugh?