The story of the legendary submarine "Papanikolis" and its activity during the Greek-Italian War. The experienced and patriotic commander, Captain Miltos Iatridis, his right-hand man, Lieutenant Vasilis Aslanoglou, and their dedicated crew, patrol the Strait of Otranto. Their dangerous mission is to seek and destroy an Italian naval convoy, and even though they are outgunned, they manage to sink three ships. As a result, the entire Italian fleet of the Adriatic Sea pursues "Papanikolis"; however, the stealthy submarine manages to return safely to its base.