In 1945, with Korea under Japanese colonial rule and World War II reaching its peak, Jazz musician Lee Gang-ok travels with his young daughter and band members to the Japanese mainland, where they are duped and forcibly taken by the Japanese military to the small island of Hashima. There they become conscripted labourers in a massive undersea coal mine that supports the Japanese war effort. As the months pass and the U.S. increases its military measures on Japan, tensions reach a breaking point between the cruel Japanese overlords and the exhausted and desperate Koreans. Meanwhile, Park Mu-young, a U.S.- trained independence fighter on a covert mission, unveils the dark reality behind how the island is actually run. Just as chaos is about to break loose, and the Japanese leaders begin to consider drastic measures, Mu-young steps up with a daring and risky plan to evacuate all 400 Koreans from the island.