The Secrets of the Coastal Settlement

In the settlement by the Black Sea, where the characters of the book Yurko, Kostya, Olga, and Dimka met and became friends, new vacationers appear - a father with his daughter Marina. Their behavior raises suspicion in Yurko, a connoisseur... of books and films about spies and espionage.
The guys begin to watch the newcomers, not realizing that this is the beginning of exciting adventures, as a result of which they will not only uncover Marina's father's secret but also learn a lot of interesting things about the history of espionage from ancient times to the present day, the lives of outstanding spies, and what qualities one must possess to succeed in such a challenging field.