One day, while playing at the park, five-year-old Shinnosuke Nohara and his friends from the Kasukabe Defense Force are approached by a young woman named Tamiko. She arrives through a time machine and makes an astonishing claim: she is Shinnosuke’s fiancée from the future. Tamiko explains that twenty years from now, the adult Shinnosuke has been turned into a stone statue by her father, the powerful business tycoon Masuzo Kaneari. Before his petrification, adult Shinnosuke told her to find his five-year-old self, as only his younger self can save him. Intrigued and bribed with future snacks, Shin-chan and his friends Kazama, Nene, Masao, and Bo travel with Tamiko to the future.

The future they arrive in is a stark and dystopian world. Years ago, a massive meteorite collision cast a permanent veil of dust over the Earth, blocking out the sun and plunging the planet into an endless, dark twilight. In this chaotic environment, Masuzo Kaneari, the president of Kaneari Electric, consolidated power by providing energy security, becoming the de facto ruler of Neo Tokio, a glittering metropolis built amid the ruins. His ruthless corporate empire has destroyed all competition, including the company where a now-bald and defeated Hiroshi Nohara once worked, while a stressed and obese Misae Nohara remains at home.

The group learns that adult Shinnosuke, who retains his carefree and defiant nature, quit working for Kaneari, an act of insubordination the power-hungry mogul could not tolerate. In revenge, Kaneari used a beam to coat him in stone. He then plans to force his own rebellious daughter, Tamiko, to marry his compliant assistant, the adult version of Kazama, using the petrified Shinnosuke as leverage. To rescue the adult Shinnosuke and stop the wedding, the five-year-old Shin-chan and his friends, with the help of their older, future selves—including a scientist Bo, a teacher Nene, a shopkeeper Masao, and a police officer Himawari—infiltrate Kanearis fortress. After a series of chaotic battles involving the Bride Corps, Kanearis all-female security squad, the young Shin-chan locates the stone statue of his adult self.

In the climactic finale, adult Shinnosuke is reanimated. Together, the two Shinnosukes combine their unique Obaka power, a supernatural energy born from their absurd and nonconformist nature. This combined force is strong enough to part the dust clouds in the sky, letting sunlight return to Earth for the first time in years, which simultaneously breaks Kanearis grip on power. With the future saved and Kaneari arrested, young Shin-chan and his friends return to their own time, relieved to find their world still bright and their parents unchanged. The film concludes with a glimpse of the adult Shinnosuke enjoying the sunshine he helped bring back, fulfilling his simple wish to see the world bright again.