Inspector Koichi Zenigata investigates a wave of Lupin III impersonators after one is arrested for shoplifting. As numerous Lupin impostors swarm Tokyo, causing mass confusion, Zenigata returns to hunt the real thief but is overwhelmed by identical-looking suspects, complicating his efforts.

During the chaos, Zenigata encounters Yasuo, a pickpocket who adopted Lupin's identity after an elderly man (implied to be the real Lupin) gave him a green jacket and Walther P38. Yasuo escalates his crimes, targeting the "Ice Cube" diamond prototype. Zenigata observes Yasuo battling a rival red-jacketed Lupin impostor, both vying for recognition as the genuine article.

Zenigata learns the Ice Cube is a prototype for new nuclear warfare technology. Recognizing the greater global threat of its potential misuse, he deliberately allows Yasuo and the red-jacketed impostor to escape with the device. This decision prioritizes preventing catastrophe over immediate capture, demonstrating pragmatic judgment. The outcome of the duel between the impostors remains unresolved, but Zenigata continues pursuing the victor, unwavering in his dedication to capturing Lupin despite the lingering impostor crisis.