Sickan, the leader of the criminal Jönsson gang is incarcerated in a mental institution, attended by Dr M.A. Busé, who admire a handbook in crime that Sickan has been writing. Blackmailed by the nefarious banker Wall-Enberg, Dr Busé and Sickan's partners Vanheden and Dynamite-Harry execute Sickan's plan of stealing the valuable Black Diamond of the Romanovs from the National Museum.

The Jönsson Gang & the Black Diamond

When Vanheden and Dynamit-Harry tries to break doctor Busé out of jail, they get Herman Melvin instead. He has been working at the Swedish Mint and the doctor, who has started to think like Sickan, has written a plan for making a burglary there, on Melvin's back. However, something goes wrong. They don't get a lot of Deutsch-marks as they planned but instead, they end up on a ship going to Poland, together with bunches and bunches of Polish Zlotys (the Polish currency). In Poland, they discover that the mafia are very interested in that money and Wall-Enberg is working with the mafia.

The Jönsson Gang's Greatest Robbery

Vanheden and Dynamite-Harry are approached by Sickan's grandmother and Sickan's younger brother Sven-Ingvar. Their evil nemesis Wall-Enberg are planning to steal billions of euros of the European Union's grants to culture. The grant of 350 million kronor, paid by an Italian bank acting as intermediary, equals 60 billion lire but through assistance of a Sicilian "businessman", this sum becomes 60 billion kronor when transferred. The money is to be transferred to a large number of bank accounts, and the account numbers are saved on a floppy disc that the Jönsson gang is trying to steal.