Plot
Doraemon is sleeping in his closet as a robotic hand emerges, reaches for his bell, and absconds with it. In a panic, he consults Nobita, who points out that the bell is broken and asks why it matters. Without telling the truth, Doraemon claims it is valuable to him and recalls a time when he asked the factory where he was made if they could repair the bell. A manufacturer explained that it is an obsolete model and too expensive to fix, but Doraemon insisted on having it repaired. He pulls out the Sherlock Holmes Set (シャーロック·ホームズセット) and gives it to Nobita, who uses it to discover the name of the culprit: Kaitō DX, a well-known gadget thief in the 22nd century.
After discussing the incident with Dorami, Doraemon and Nobita—along with Shizuka, Gian, and Suneo—travel to the 22nd century to visit the Secret Gadget Museum, where they meet Director Fiks and Kurt, a young museum guide giving the group a tour. Doraemon and Nobita search for the lost bell and meet Inspector Mustard, who is investigating a strange case involving the relationship between Kaitō DX and the museum. Gian and Suneo unknowingly pass through the Gulliver Tunnel (ガリバートンネル), shrinking to a tiny size. Nobita, Doraemon, and Shizuka get lost in a tunnel leading to the Sun-Manufacturing Machine Floor that powers the museum. Professor Peppler's past negligence caused a freak accident that made the artificial sun gradually expand into a giant star and endanger the world, leading to his dismissal. During that incident, his colleague Harutoman discovered "Fullmetal", which enabled him to stabilize the sun's core and subsequently manufacture secret gadgets.
Doraemon begins acting more and more like a feral cat. The group spends the night at Kurt's home, where they meet his gelatinous robot pet, Popon. The next day, the museum receives a threat from Kaitō DX, who vows to steal four gadgets at noon. Kaitō defeats the group and steals the listed items. Using the Holmes gadget, Nobita discovers that the targeted security gadgets were all repaired on the exact day Peppler was captured. The group searches for him, leading them to a secret passage. When Kaitō intercepts them, Nobita uncovers his real identity: Kurt himself. Kurt explains the invention he and Peppler were trying to complete. Converting Fullmetal into "Peppler-Metal" requires six hidden chips that Peppler stashed inside various gadgets, including Doraemon's bell. To retrieve these items, they created the persona of Kaitō DX.
Still shrunken, Gian and Suneo retrieve the Big Light (ビッグライト), restoring themselves to normal size. Peppler recovers the chips and begins the conversion. This causes all Fullmetal gadgets to disappear and destabilizes the artificial sun, releasing a monstrous protector that attacks the group. After transforming into Kaitō Dora DX by putting on Kaitō's Bow, Doraemon defeats the monster but accidentally destroys the mainframe controlling the sun. Nobita orders Popon to swallow the sun, making it disappear. As the museum collapses around them, Peppler reverses the conversion to restore the gadgets, allowing Doraemon to use the Restoring Beam (復元光線) to repair the building and save the team. Kurt helps search for the bell and returns half of it to Doraemon. In the end, Nobita finds the other half, realizes why Doraemon cherishes it—because it holds the memory of when their friendship began—and brings it to him.
