At the start of WWII, Salvatore Todaro commands the Italian Royal Navy submarine Cappellini. One dark night in October 1940, while navigating in the Atlantic, he comes upon an armed merchant ship sailing with lights out. He shoots his cannons at the ship and sinks it. At that moment, the Commander makes a decision that was destined to go down in history: he saves the 26 shipwrecked Belgians who otherwise would have drowned in the middle of the ocean and disembarks them at the nearest safe harbor, as prescribed by the law of the sea. To make room for them on board his submarine, he is forced to navigate on the surface of the water for three days, visible to the enemy forces and endangering his life and that of his men.