At the local department store, the precocious kindergartener Shinnosuke Nohara, known to everyone as Shin-chan, convinces his mother Misae to buy him the latest toy from his favorite superhero, Action Kamen. He is thrilled to receive the Action Sword, but when he opens the box at home, he finds nothing but a simple ruler. While this disappointment is fresh, the Nohara family's world is about to change in far more drastic ways. Unbeknownst to them, this seemingly worthless ruler is actually one of two legendary treasures, the Golden Spear, that holds the key to an ancient prophecy.

The peaceful town of Kasukabe is the target of a sinister invasion. The evil ruler Asse Dak Dark, the emperor of the dark world known as Don Kurai, has set his sights on conquering the human realm. He has already stolen one of three sacred treasures needed for his plan, the Copper Dotaku, and seeks the remaining two: the Golden Spear and the Silver Shield. To stop him, a mysterious man sends the Spear and Shield to the human world where they await a chosen hero.

Not long after the toy store incident, Shin-chan finds a black dog who is a mirror image of his beloved pet, Shiro. He brings the stray home, names him Kuro, and despite his mother's protests, convinces his family to let him stay. That same night, Shin-chan is awakened by a beautiful woman named Puririn who asks for his help. He naively follows her, and by opening a door for her, he inadvertently creates a portal that connects the human world to the dark world of Don Kurai. Darkness begins to seep into Kasukabe, causing strange and chaotic calamities that only Shin-chan seems to notice.

Soon, a mysterious and energetic young girl named Mata Tami appears before Shin-chan. She can transform her shape and possesses incredible powers. Mata explains that Shin-chan is the chosen hero destined to wield the Golden Spear and defeat the emperor of Don Kurai. However, Puririn is not finished with her deception. Using her charms, she tricks Shin-chan into believing Mata is the enemy, and he helps Puririn seal Mata away in a small box. The world around him begins to crumble as the illusion is broken, and Shin-chan realizes his terrible mistake.

With the help of his family, who finally see the truth, Shin-chan frees Mata. Together, the Nohara family launches a daring assault on the dark world of Don Kurai to confront Emperor Asse Dak Dark. In the final battle, the black dog Kuro reveals his true nature, transforming into the legendary Silver Shield. Shin-chan then takes the ruler he bought earlier, and it finally reveals its true power as the Golden Spear. Armed with the two treasures and joined by his parents and his baby sister Himawari, Shin-chan fights to banish the darkness, restore peace to Kasukabe, and fulfill his unexpected destiny as a hero.