During the Second World War, the U-boats were the most powerful weapon of the German Wehrmacht. Every day, they sent several tons of Allied materiel to the bottom of the ocean. But life aboard these submarines was very harsh. With few conveniences and little privacy, the sailors needed to find ways of distracting themselves.

Young Josef has found a way to pass the time. Whenever he has the chance, he chats with his new friend, a seal — debonair, philosophical, and apparently invisible to everyone but Josef. With him, Josef reminisces about his childhood in Germany, when he and his brothers would secretly spy on Emma, the prettiest girl on the Rue des Chiens Marins…