For the third year, the expedition has been searching for the causative agent of an infectious disease rampant in the Ussuri taiga, where a major construction project is about to begin. Meanwhile, the enemy seeks to destroy the fruits of years of labor of Soviet microbiologists. Young scientist Vasily Zheludev exposes a master saboteur and forces him to flee into the taiga.

Directors: Aleksandr Alekseev, Yevgeniy Alekseyev