Secret testing of a new aqualung, elaborated by a soviet scientist Eugenie Ivanovich Sretensky is held on one of the Northern fleet naval vessels, not far from the frontier. The testing area is closely protected by frontier guard. Best divers from all fleets are gathered for the aqualung testing, their state of health is observed by an experienced doctor, testing is headed by E.I..Sretensky. The unique possibilities of a new underwater respiratory device, surpassing all existing prototypes, arouse a deep interest of western secret services. So do Sretenskys diaries, containing all testing stages. A young post-graduate of a medical institution Balashova comes from Moscow under the pretence of a doctors assistant to take away the diaries.