At the beginning of the Second World War, Salvatore Todaro is in command, very much in his own way, of the submarine Cappellini of the Italian Royal Navy. In October 1940, while sailing in the Atlantic, the silhouette of a merchant ship with its lights off looms in the dark of night. This is the Kabalo, which it would be discovered later, was a Belgian ship, but carrying British weapons and munitions and which suddenly opened fire on the submarine and its Italian crew.A brief but violent battle ensues which ends when Todaro sinks the merchant ship with cannon fire. And it is at this point that the commander reaches a decision that is destined to make history: to rescue the 26 Belgian survivors, who would otherwise have been condemned to drown in the middle of the ocean, and to disembark them at the nearest safe harbour, as required by the law of the sea. To take them all on board, he is forced to sail on the surface for three days, making himself visible to the enemy, risking his own life and those of his men.