Nobita [Megumi Oohara] is daydreaming about catching and solving the mystery behind Kaito Lupin as Sherlock Nobita, but he slides on the balcony and snaps out of his daydream. He ends up in the classroom caught in a conversation with Suneo[Tomokazu Seki], Shizuka[Yumi Kakazu], and Gian[Subaru Kimura], leaving Nobita alone as usual.
Back at Nobita's house, Doraemon[Wasabi Mizuta] is sleeping in his closet when a robotic hand appears at the front of the computer and leaves a mysterious “DX CARD” in Nobita's room. He crumples the test paper in his bag after noticing the card, just as his mom calls him to eat in the kitchen. While they leave, Doraemon remains in his closet, and the bell around his neck is snatched by the robotic hand and slips away through a time hole. Doraemon panics about the missing bell and begs Nobita for help.
Nobita asks why the bell matters, and Doraemon traces its origin to the factory where it was made, only to learn it is now dismissed as substandard junk. Frustrated, Doraemon removes the bell and Nobita proposes a workaround: if the bell is the problem, we’ll replace it with a gadget. He retrieves the Kisekai Camera from the Pocket and tries several items that resemble the cat bell—a Christmas bell, a gondola bell, a banana, chirps, an egg yolk, and even a cough neck. When Nobita sees Doraemon act like a cat, he realizes that if something is lost, you can try that gadget. Doraemon then pulls out the Sherlock Holmes Gadget, and together they map a path to recover the bell and uncover the thief.
With help from Dorami, the duo ventures to the 22nd century to the Gadget Museum, where they meet Director Fiks and Kurt — a young guide of the museum. Kurt gives them a tour of remarkable inventions, and the group is awed by many devices, especially the very first Anywhere Door. While exploring, Doraemon and Nobita search for the bell, which leads them to a pond and a “Baba Robot.” A robot accidentally slips into Doraemon's pocket, making him momentarily unable to use his pockets.
Fascinated by the gadgets, Gian[Subaru Kimura] and Suneo[Tomokazu Seki] shrink through a shrinking tunnel, while Nobita, Doraemon, and Shizuka[Yumi Kakazu] are also reduced, venturing into a zone called the Natural Man Made Solar Sun that powers the museum. A freak accident, caused by negligence from Dr. Pepura Hasake (who secretly lives in a hidden room with his granddaughter Ginger), destabilizes the solar core and puts the world in danger. This crisis leads to the dismissal of the museum director and pushes forward Dr. Harutoman’s discovery of a metal known as Fullmetal, which can stabilize the sun and earns him fame.
Doraemon begins to feel more feral, and the group ends up spending a night at Kurt's home, where they meet his pet Poppon. The next day, the museum receives threats from Kaitou DX to steal four gadget items at noon. Even with tightened security, Kaitou defeats Doraemon and steals the listed items. Nobita, aided by the Sherlock gadget, deduces that the security context was amended on the same day Dr. Pepura was captured, sending them on a secret path to uncover the truth. They encounter the Head of Gorgon, who attacks with a laser that petrifies anyone struck by it. Shizuka and Nobita counter by turning the head to stone with a mirror, and Nobita recovers Kurt’s monitor in the process.
Kaitou is revealed to be Kurt himself—a Copy Robot who took his place. Kurt explains that he and Dr. Pepura planned to convert all Fullmetal into Pepura-metal and recover the chips hidden under six lost items during repairs. To recover them, they created Kaitou DX and spread the rumors to pursue their goal. The truth triggers a turn of events as Dr. Pepura recovers the chips and resumes the process, while Kurt tries to outrun the others.
Shrinking giants Gian and Suneo eventually squeeze through a door to recover the Magnificent Light and restore themselves to normal size. The plan hits a critical moment when Pepura’s project begins, and a monstrous protector breaks free to attack as the man-made sun becomes unstable. Doraemon, wearing Kaitou Dora DX, fights the security monster, but the mainframe controlling the sun is damaged. In a bold move, Nobita orders Poppon to absorb the sun, helping to collapse the threat. Dr. Pepura reverses the process, converting the metal back and mending the system. Doraemon and Kurt repair the museum with the recovered torch and the magnificent lamp, and they come to understand why Doraemon cherishes his bell: it holds the memories of Nobita and Doraemon's enduring friendship.
