A scientific expedition remains in the taiga for the winter. Encrypted materials of important military significance are sent with engineer Sapov, accompanied by former partisan Oloni, who is to put the engineer on a plane. At one of the stops, Sapov is unexpectedly visited by Inspector Petrov, who is in fact a former White Guard soldier named Yaniga and currently serves in Japanese intelligence. The saboteur's goal is to steal secret documents and flee abroad on this plane. He kills Oloni and recruits Sapov, taking Oloni's son, the pioneer Gaichi, as his guide. The boy, realizing that he is dealing with a fierce enemy, manages to inform the Nanai people, and the border guards who arrive on the scene detain the criminals.