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.

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.