When Mats and David are swimming in the harbor at the outskirts of Stockholm, some other boys take their clothes, but drop them after some hundred meters. Mats' trousers have become dirty of oil, but he finds another pair of trousers on the quay. To his big surprise he discovers that he can take one banknote after another, in an endless stream, from the right pocket of his new trousers. He and David go on a shopping spree, but when they involve the girl Livli in their secret, they understand that they should use the money for more noble purposes. They start by helping the homeless man Zeke, and continue by sending 50 million crowns to various aid organizations. From the TV News they learn that a similar sum of money has disappeared in a mysterious way from the banks, and understand that there must be some connection. The whole bank system is on its way to collapse, so the police are eager to seize the children. Also the arms dealer William Otto is looking for the magic trousers, which once made him rich. When he finds out that the children have them, he captures and tortures Mats' father, and offers to release him in exchange for the magic trousers.