Shinnosuke Nohara, a five-year-old boy nicknamed Shin-chan, becomes the unexpected linchpin in an interdimensional crisis when Action Mask, a hero from another realm, recruits him to recover the stolen Action Stone—a vital artifact enabling travel between dimensions. Thrust into this role after the stone’s theft by the antagonist Haigure Maō, Shin-chan’s journey takes a pivotal turn when he impulsively swallows a second Action Stone concealed within a candy wrapper, binding his fate to the artifact’s power. This accidental act strands his family in a parallel world during a beach outing, where they face humans warped by Haigure’s control.

Equipped with a weaponized tricycle and aided by his talking dog Shiro, Shin-chan confronts Haigure’s forces, including the Rope Ladies and Tea Pack Man, blending chaotic ingenuity with fearless resolve. His unorthodox tactics—often accidental yet effective—peak during the climactic showdown, where he snatches the original Action Stone from Haigure and summons Action Mask for reinforcements. Their combined efforts unleash a decisive beam attack, neutralizing the threat.

Post-victory, Shin-chan earns a replica of Action Mask’s costume, symbolizing his pivotal role. His interactions with allies like Ririko and Dr. North Kasukabe, alongside navigating betrayals such as Ume Matsuzaka’s, showcase his adaptability amid shifting loyalties. The narrative centers solely on this adventure, framing him through traits like mischievous curiosity and spontaneous courage—evident when he mistakes interdimensional chaos for a movie set—while leaving his broader history unexplored. His actions, driven by chance and stubborn determination, cement him as an accidental hero resolving cosmic upheaval through tenacity and serendipity.