Pyotr Semenovich - instrument engineer, falls into 1876 - the eve of the last Russian-Turkish war. He remembers the level of technical progress only from the film "Turkish Gambit." But he is a Russian man and does not want to sit idly by. Moreover, some information about the history of technology has been preserved in his memory. Should he not aim at creating a small submarine, because he has an idea of ​ ​ the reasons for the modest success of mine boats of this period? The "little diver," disguised as a dolphin, causes a stir in the Black, Mediterranean, and Japanese seas, because he carries weapons on board, from which no warship can escape.