 On November 27th 1942,the French submarine "Casabianca", under the command of her daring captain Commandant Jean Lherminier, manages to evade the Germans  eye and to escape from Toulon harbor. The film describes life on board, the dangers gone through in common by the valiant crew as well as the struggles of Corsican patriots. "Casabianca" closes on September 9th 1943 with the arrival of the rebellious submarine in Ajaccio where she lands a battalion of freedom fighters all the way from Algiers, announcing the liberation of the country.