Kondoo Isamu III, grandson of the leader of the Shinsen Group of Tokugawa loyalists who fought the New Meiji Government during the collapse of the Shogunate, hires Lupin and company to recover the legendary Golden Cannon, sunk by the Shogunate forces in the Tsugaru Strait (between Hokkaido and Honshu), and which would be worth some 7-8,000,000,000¥ to collectors. The problem is the violent deep-sea current which protects the sunken vessel. A ring toss catch project acquires the golden cannon, but afterwards a death struggle for possession takes place. Okida, controlling a monster squid, takes a do-or-die course of action, shouting that he will take his ancestors' legacy and make Hokkaido an independent country.