The story is set in a dystopian world in which the gap between the poor and the rich has been spread even wider because of a pandemic. The protagonist, Isamu Kouyama, is a poor young man who has finally found employment, though it's one with harsh conditions. When he was almost robbed of his paycheck, he decided that he needed to learn how to defend himself. After he told this to Le Anh Cong, his former co-worker and a foreign technical intern trainee, he began learning the Vietnamese martial art of Vovinam. Later, when his younger sister's lover, Koji Iwanami, was accused of a crime and needed Isamu to help prove his innocence, Isamu decided to take the chance to help and to make a quick fortune by signing up for a volunteer job on a luxury cruise ship... However...!