Gaira, a humanoid sea beast spawned from the discarded cells of Frankenstein's monster, attacks the shores of Tokyo. While the Japanese military prepares to take action, Gaira's Gargantua brother, Sanda, descends from the mountains to defend his kin. A battle between good and evil ensues, leaving brothers divided and a city in ruins.

Sawaki is released from prison after serving eight years for murdering a rival gang leader in order to obtain the land rights to a landfill for the Kotaki clan and he discovers that the world has changed significantly in that time. The police have forced the disbanding of all yakuza groups in and around Tokyo and most of the former yakuza have moved on to legitimate jobs. Sawaki is disgusted that his yakuza brothers have abandoned their codes of honor and are abusing their newfound power in the legitimate business world to exploit the poor. Sawaki feels partly responsible for making them so powerful and enabling their corruption, so he decides to follow his traditional code of honor to save the residents of a small town where his former lover lives.