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 siblings Niklas and Anneli must accompany their slightly giddy aunt Tinne to the Zoo in Copenhagen. The visit to the zoo has quite different consequences than Aunt Tinne has ever dreamt of. When the children win a real pony in a lottery they have to bring it home to Stockholm without anyone noticing.
The movie begins with the Jönsson gang making a minor burglary. It goes wrong and the gang leader, Sickan, gets caught. After he has spent 10 months in jail, Vanheden and Rocky come to welcome him when he's released. But he doesn't take notice of them, and he's picked up by the mysterious banker Wall-Enberg Jr. instead. He wants Sickan to perform a burglary in Switzerland, in order to retrieve the famous Bedford Diamonds. Sickan does this but is fooled by Wall-Enberg and nearly gets caught. When he returns to Sweden and re-unites with his old gang members, he's set on revenge, both physically and psychologically...