"Gegege no Kitarō: Saikyō Yōkai Gundan! Nihon Jōriku!!" is a feature-length anime film based on the long-running "Gegege no Kitarō" series, which revolves around the adventures of Kitarō, a yōkai boy who acts as a mediator between the human and supernatural worlds. The film centers on a large-scale conflict involving yōkai and humans, set against the backdrop of Japan’s modernization and the encroachment of human development on yōkai territories.

The story begins with the arrival of a powerful yōkai army led by the enigmatic and formidable Nue, who seeks to reclaim Japan for the yōkai by driving humans out. Nue’s forces, known as the "Strongest Yōkai Army," are composed of ancient and powerful yōkai who have grown resentful of humanity’s expansion and disregard for the natural world. As the yōkai army begins its assault, causing widespread chaos and destruction, Kitarō is called upon to intervene and restore balance.

Kitarō, accompanied by his loyal companions—including his father Medama-Oyaji, the rat-like Nezumi-Otoko, the one-eyed Ittan-Momen, and others—must confront the yōkai army while grappling with the moral complexities of the conflict. The film explores themes of coexistence, environmentalism, and the consequences of progress, as Kitarō struggles to find a solution that protects both humans and yōkai. Along the way, he encounters new allies and adversaries, including a human scientist who plays a pivotal role in understanding the origins of Nue’s wrath.

The narrative builds toward a climactic battle where Kitarō must confront Nue and his army, testing his resolve and abilities as a mediator between worlds. The film balances action, humor, and emotional depth, staying true to the spirit of the "Gegege no Kitarō" franchise while delivering a self-contained story that appeals to both longtime fans and newcomers. The resolution emphasizes the importance of harmony between humans and yōkai, highlighting the need for mutual respect and understanding in a rapidly changing world.