The Baron and his cronies steal a precious crown and hide in a secret room of an abandoned castle. With the help of their tiny robot Robin, Bassie and Adriaan set out to find the crown and return it to the museum.

Bassie finds a mysterious stone while on holiday in Greece. Three crooks are very interested in the secret the stone holds.

The Baron, B2 and Vlugge Japie have been in prison since the previous adventure. Now that they have escaped, they want to take revenge on Bassie and Adriaan. They steal a valuable crown from a museum after escaping from prison. They hide in a secret room of an abandoned castle. The crooks want a large amount of ransom for the crown, otherwise they will have it melted down, and they then lure Bassie and Adriaan into a trap. However, the duo escapes. But then it turns out that the baron accuses them of stealing the ransom. So they have to find the crooks to prove their innocence. With the help of a computer program made by Robin, they find the hiding place. The crooks try to flee abroad, but on the way they drive their car into a trap that Bassie and Adriaan have set up. They are driven to the police, where they are immediately put behind bars.