One dark night during the early phases of WWII, the Italian Royal Navy submarine Cappellini sinks an armed merchant ship sailing with lights out. At that moment, its commander Salvatore Todaro makes a decision that was destined to go down in history: to save the 26 shipwrecked Belgians who otherwise would have drowned in the middle of the Atlantic Ocean and disembark them at the nearest safe harbor. 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.