The action of the novel “Captain Rumba's Briefcase” takes place at the end of the 19th century. A boy nicknamed Gvozdik, together with his adult friends, goes in search of a pirate treasure, but instead of treasure he finds something else. This discovery gives impetus to further entertaining events. “I won’t do it again” - these are not the usual words of a guilty boy, but the name of a sailing ship that was once commanded by Captain Foma Ivanovich Sundukov, aka Thomas John Sundukker.