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.

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.

Mrs. Blank visits her lawyer Karels to request his assistance with a matter. She is a niece of Baroness de Hagen. The baroness's son and daughter-in-law have passed away, and Mrs. Blank would be the sole heir, provided that the young son is no longer alive. Karels has to make sure the son disappears so that Mrs. Blank can claim the money in ten years. The son grows up in a gypsy family, but one day he finds out that this is not his family. What will happen? Will Mrs. Blank still get her money?

Pietje Bell, the famous boy from Rotterdam, goes chicken hunting with his friends Peentje and Engeltje, but they are pursued by a police officer who, despite Aunt Cato's protests, takes them to the police commissioner. Pietje and his friends manage to talk their way out of it. Pietje performs a circus show with his friends Peentje and Engeltje, which is disrupted by a fire when one of Pietje's tricks goes wrong.

Johan, Hilde, Alwin, and Robert are walking through the forest. Suddenly, they see a man being beaten up by a few knights. They rush to the man's aid and chase the knights away. The man, named Casper, introduces himself and says that Knight Koen sent him. He has a message for Knight Bruno. The four take Casper to Bruno's castle, where he tells Knight Bruno his message: the green seal has disappeared. It has been stolen, and they suspect the black knight. Johan and Alwin set off together to retrieve the seal. But is Casper telling the truth? And was he really sent by Knight Koen?

George and Jimmy time-travel to the 18th century and find themselves on a deserted pirate island where the inevitable buried treasure lies buried.