In 1944, with Korea under Japanese colonial rule and World War II reaching its peak, the bandmaster LEE Gang-ok earns a meager living in Seoul despite a ban on jazz music. However when he tries to travel with his young daughter and band members to the Japanese mainland, they are duped and forcibly taken to the small island of Hashima, located 15km off Nagasaki. There they become conscripted laborers in a massive undersea coal mine that supports the Japanese war effort.

Hashima (nicknamed “Battleship Island” for its resemblance to a war vessel) is a place of unspeakable cruelty for 400 Koreans who suffer under slave-like conditions and almost no pay. These include CHOI Chil-sung, a notorious Seoul gangster; Mallyon who has been forced into sexual servitude as a so-called “comfort woman”; and a mysterious young man named PARK Mu-young. Gang-ok, meanwhile, manages to protect his daughter Sohee by serving as an interpreter and dealmaker between the Koreans and Japanese.

As the months pass and the U.S. steps up its bombardment of Japan, tensions reach a breaking point between the cruel Japanese overlords and the exhausted and desperate Koreans. Meanwhile PARK Mu-young, who turns out to be a U.S.-trained independence fighter on a covert mission, uncovers some dark secrets about how the island is really run. Just as chaos begins to break loose on Hashima, and the Japanese leaders consider drastic measures, Mu-young steps up with a daring and risky plan to evacuate all 400 Koreans from the island.