Three boys—brothers Mladen and Željko and their friend Ivica—are devoted fans of Zagreb’s Dinamo Football Club. They win the grand prize in a sports lottery. They keep it a secret from their parents because they want to use the money during their upcoming summer vacation at the seaside to buy a small boat and embark on an adventure playing pirates, unsure whether their parents would approve. Criminals Tigar and Žuti want to get their hands on the boys’ money, and they enlist the help of their very young accomplice Duško, nicknamed Žabac, who becomes close with Mladen, Željko, and Ivica. On the train journey to the sea, the boys are accompanied by their teacher, Vera.