A black, mysterious boy appeared in Koko's class. He doesn't hang out with his colleagues, he doesn't play football, he knows a lot, and he doesn't answer any questions at school. Something is wrong with him. Koko, who has a nose for real secrets, begins to doubt him. And yet, he still has no idea what he's gotten himself into...

On Koko's order and for "paycheck", Koko's colleague Marijana starts stalking Marko Lukarić. Her wish is to become the new Mata Hari anyway. In fact, she works for both sides, as befits a true spy.

Koko decides to go straight to the "enemy" camp, to visit the unusual aunt Vera, who solves crossword puzzles and who...

Double spy Marijana Hari discovers Marko Lukarić has a new letter from Munich, from his parents - who are not there. Koko devises a way to get hold of those strange letters.

On the island, our detectives discover the notebook of Marko Lukarić - from Split. They return to the island at night, and a storm is brewing.

Koko and the team come to Aunt Vera in the hope of finding Mirko. Mirko has escaped, and Koko has dangerously little time to solve the case.