Petr Semenovich is an instrument engineer who finds himself in 1876 - the eve of the last Russian-Turkish war. He remembers the level of technological progress only from the film “The Turkish Gambit”. But he is a Russian man and does not want to sit idly by. Moreover, some information about the history of technology was preserved in his memory. Shouldn’t he be aiming to create a small submarine, because he has no idea about the reasons for the modest successes 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 on board a weapon that no warship can escape.