This book is the final part of the trilogy, the first two parts of The Secret of the Leather Briefcase and The Mystery of the Ancient Statuette were published earlier. But this story can be read as an independent work. "Big Watermelon" - a rare banknote, which is worth millions. A fanatical collector, his young neighbor, as well as an American businessman (a former thug and racketeer from the time of perestroika), and a ruined entrepreneur, the son of a KGB colonel, converge in the struggle for its possession. Young spouses from Moscow and their Japanese friends are trying to stop the criminals. Who will get a unique rarity?