Pyotr Semenovich is an instrumentation engineer. He finds himself in 1876—the eve of the last Russo-Turkish war. He remembers the level of technical progress only from the film “The Turkish Gambit.” But he is also a Russian man, and he won’t sit with folded hands. Especially since certain details about the history of technology have remained in his memory. So why not attempt to build a small submarine—since he knows the reasons behind the modest successes of the mine-craft of that period? A “Little Diver,” disguised as a dolphin, causes a commotion in both the Black Sea, the Mediterranean, and the Japanese Sea—because it carries weapons that no warship can escape.