Nobita throws a temper tantrum over wanting a large RC toy robot to upstage his friend, Suneo, who has just been showing off a new robot made by his cousin. The outburst angers Doraemon, who uses his Anywhere Door to escape the summer heat at the North Pole. Nobita soon follows and discovers a strange bowling-ball–like orb that starts blinking with a pulsating light, summoning what looks like a giant robot’s foot. He uses the foot to sled down, crashing into his room through the Anywhere Door, and the orb follows him home, bringing another robot piece into the backyard.
Doraemon admits he has nothing to do with the robot parts, and the two of them, using the Opposite World Entrance Oil and the Roll-Up Fishing Hole, slip into the World Inside the Mirror—an alternate mirror world without people. There they construct a robot Nobita dubs Zanda Claus, convinced the sphere is from Santa Claus. With a brain-wave controller Doraemon pulls from his pocket, Nobita makes the robot perform gymnastic maneuvers in the mirror world and even invites Shizuka to join the fun. The trio enjoys their play until Shizuka accidentally presses a panel that unleashes a colossal laser beam, destroying a skyscraper and instantly revealing how dangerous Zanda Claus can be.
Realizing the threat, they decide to return to the real world and forget about the robot. Yet Nobita has forgotten the sphere that has been silently sending telepathic messages to a mysterious girl named Riruru, the true owner of Zanda Claus. When Nobita slips up, Riruru tracks him down and forces him to reveal the robot’s location. She takes the Roll-Up Fishing Hole from Doraemon’s spare pocket and, with Nobita in tow, returns to the World Inside the Mirror to reclaim Zanda Claus. After a tense moment, Nobita borrows the fishing hole again to a forested area near Mt. Ura, spurred by two successive shooting stars.
Following another shooting star through the unrolled Roll-Up Fishing Hole, Nobita discovers Riruru building a massive robot army. Doraemon, suspicious of Nobita’s odd behavior, follows and they both enter the mirror world to find a sprawling base where Riruru orders the robots to work faster, revealed to be a human-hating machine. When she catches them, Nobita and Doraemon escape back to their own world, but the portal is accidentally damaged by Zanda Claus. The sphere at home wakes up and bounces around until Doraemon uses Translation Jelly to understand it; it reveals itself as the “brain” of Zanda Claus and warns that a giant robot army seeks to conquer Earth.
Desperate, they attempt to alert authorities, but few believe them. Only Suneo and Gian stand by Nobita and Doraemon. Doraemon creates a plan: he places the sphere in a special incubator, hatchling a yellow chick named Pippo. They reenter the Mirror World through Shizuka’s bathtub using a special oil, but they leave Shizuka out of the mission due to the risks. They are soon captured by the robot army, with Pippo and Nobita managing to escape.
Nobita resolves to help his friends and heads to the base to rescue them. There, they see Zanda Claus being disposed of after its role is revealed. Pippo enters Zanda Claus to create a distraction, while Nobita frees his friends. Shizuka, meanwhile, learns about the Mirror World and goes to help an injured girl she finds there, bringing her back home. Afterward, Shizuka uses Doraemon’s first-aid kit to tend to the injured Riruru, which helps Pippo earn Nobita’s trust and deepens the bond within the group.
Riruru carries deep trauma and distrust toward humans, and the only person she feels connected to is Pippo, whom she repaired after a robot mishap. Despite the group’s efforts, Riruru escapes and tries to warn the robot army about the Mirror World’s existence. Nobita and friends locate her before a major confrontation; during a heated moment, Riruru fires a laser from her finger at Nobita, but Pippo jumps in front of him and is badly wounded. This becomes a pivotal moment for Riruru, prompting her to reconsider and help the humans, though she won’t reveal the reasons behind her wariness of humanity. The robot commander locks her up, but Nobita, Doraemon, and the others arrive with Zanda Claus to rescue her.
Back in the real world, Riruru is locked up in a birdcage with the Small Light. The robot army grows suspicious of humanity’s absence and discovers the truth that they are in a fake world by analyzing satellite images and comparing them to real-world data. The group heads to the lake where they first entered the Mirror World, and they intercept the army there. While Shizuka stays behind to talk with Riruru, she hatches a plan: she re-enlarges Riruru with the Enlarging Torch Light, and the two use a Walk-In Mirror to return to the real world. They time-travel to Megatopia, 30,000 years in the past, to consult the professor who created the robots.
The professor’s aim is to remove the competition instinct from his robots and replace it with humanity and love. He collapses before he can complete the rewrite. Riruru, determined to finish the mission, continues reprogramming with guidance from the professor, knowing that she and Pippo will disappear in the process. In the present, the robot army grows stronger, and Zanda Claus sustains heavy damage while the Megatopian project nears completion. Riruru and Pippo are nearly erased as the reprogramming nears its end, but the professor’s death makes the final adjustment possible: Riruru adds the emotional influence of Nobita and his friends’ love. The reprogramming succeeds, erasing the robot army—and, with it, Riruru and Pippo.
Shizuka uses the Anywhere Door to return to Earth and reunite with her friends, though their victory is bittersweet. The next day, Nobita remains in school, pondering the fate of Riruru and Pippo. Doraemon arrives to speak before the baseball field, and Nobita wonders aloud if Riruru and Pippo might ever be resurrected. Just then, a shadow crosses Nobita’s eyes as Riruru reappears with wings on her back. The wings resemble Pippo’s form, now a giant phoenix. They visit Nobita and his friends for a moment of reassurance before vanishing into the sky, leaving Nobita hopeful about their possible return.
