During World War II, the Japanese island of Hashima (also called Gunkanjima, or Battleship Island) was the site of a brutal mining operation that used forced labour, including boys from South Korea. South Korean women on the island faced the fate of being used as sex slaves. Bandmaster Lee Kang Ok (Hwang Jung Min) and his young daughter So Hee (Kim Su An), as well as notorious gangster Choi Chil Sung (So Ji Sub), are among those shipped off to it. Park Moo Young (Song Joong Ki), an American-trained operative, infiltrates the island to rescue a fellow independence fighter, but eventually tries to help about 400 Koreans escape.