Shinnosuke Nohara, known as Shin-chan, is a five-year-old boy whose accidental immersion in an interdimensional conflict anchors the film's narrative. His character merges childlike curiosity, mischievousness, and unforeseen courage during these events.

Shin-chan's routine existence fractures when he acquires a rare Action Kamen card, catapulting his entire family into a parallel world under the guardianship of superhero Action Mask and associate Ririko. This realm faces invasion by aliens called Higures, commanded by antagonist Haigure Maô. Shin-chan inadvertently swallows the stolen "Action Stone"—a potent artifact crucial for defeating Haigure Maô—unwittingly cementing his role as the conflict's linchpin.

Throughout the adventure, his canonical traits surface: brash impulsivity, situational obliviousness, and flashes of resourcefulness. He operates a weaponized tricycle with aid from his talking dog Shiro, confronts henchmen like the "Stomach Band Ladies" and "Baron Thong," and engages in physical combat despite his age. His fearless, often chaotic actions unintentionally bolster the alien resistance.

Family dynamics echo his established persona. Interactions with parents Hiroshi and Misae, plus sister Himawari, oscillate between childish antics and latent protectiveness. Misae frequently bristles at his behavior, yet their shared mission underscores familial cohesion under pressure. Shin-chan’s loyalty to family, kindergarten peers, and teachers—who emerge in this parallel world—flickers during critical moments.

A pivotal climax sees Shin-chan reclaim the Action Stone during a clash with Haigure Maô. Empowered by the artifact, he summons Action Mask for reinforcement and joins the final battle. His contribution to the combined "Action Beam" attack helps defeat the villain, revealing his inadvertent bravery. Post-conflict, he receives a personalized Action Mask costume—a symbol of his improbable heroism.