The film is based on a true history of World War II: In 1942, a Japanese cargo ship "Lisbon Maru" carrying more than 1,800 British prisoners of war was torpedoed by an American submarine in the waters near the Zhoushan Islands. Nearby Dongki fishermen risked their lives to rescue some 300 British servicemen, and protected and rescued three Britons during a massive Japanese search. This history is known as the Lisbon Maru shipwreck.