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.

Count Andrew is away fighting a battle in the Holy Land. His brother Gilbert and his lieutenant Siegfried are quite happy that he is gone. Now we can be lazy. They are afraid that this peace will be over when Andrew returns. A messenger arrives from the Holy Land with the news that Andrew has been kidnapped. The messenger must return within three months with 10,000 gold coins, or Andrew will be killed. Gilbert and Siegfried see their chance and bribe the messenger. He tells everyone that Andrew has died, making Gilbert the new leader. Robin Hood, Andrew's son, and his mother disagree and are not willing to accept this. Will Robin Hood succeed in getting rid of Gilbert and bringing his father back?

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.

George and Jimmy travel all the way to Taskara. A rich Chinese man wants them to bring back the rare mineral OQ-4. But Taskara is inhabited by giants and, unfortunately, crooks Boris and Pedro are sniffing around too.

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?