The movie’s plot involves Nobita, who throws a temper tantrum because he wants a really large RC toy robot in order to upstage the rich kid, Suneo Honekawa, who has been showing off the new robot that his cousin made. His fit only annoys Doraemon who uses his Anywhere Door to get away from the summer heat, to the North Pole. Nobita soon follows and discovers a strange bowling ball-like orb which starts blinking with a pulsating light, and summons what looks like a giant robot’s foot. After Nobita uses the foot to sled down, crashing into his room through the Anywhere Door, the bowling ball follows him home through the door and another robot piece falls into his backyard. A frozen Doraemon follows soon after, covered in ice before being thawed out and with a cold. Learning of the robot parts, Doraemon admits to Nobita that he has nothing to do with it, and the two use the Opposite World Entrance Oil and the Roll-Up Fishing Hole to enter the World Inside the Mirror, an alternate mirror world without people. There, they build the robot which Nobita christens “Zanda Claus” as he believed the sphere summoning the parts is from Santa Claus.
Using a brain wave controller that Doraemon pulls out of his pocket, Nobita has the robot perform gymnastic maneuvers in a mirror world before bringing Shizuka Minamoto to join the fun. The trio enjoy but later however, Shizuka accidentally presses a button on the control panel that makes the robot fire a huge laser beam that destroys a whole skyscraper. The group realizes just how dangerous Zanda Claus really is, and they decide to return to the real world and forget about ever having found the robot. However, Nobita forgot about the sphere that has been sending telepathic messages to a mysterious girl named Lilulu, the actual owner of Zanda Claus. Lilulu seeks out Nobita when he accidentally lets slip all that he knows about the robot. After Lilulu proceeds to force him into showing her where it is, Nobita borrows the Roll-Up Fishing Hole from the spare pocket Doraemon keeps in the closet to take her to the World Inside the Mirror. She reclaims Zanda Claus while getting Nobita to let her borrow the Roll-Up Fishing Hole for a while.
After some time, Nobita is visibly nervous about what she is up to and, after seeing two shooting stars in a row, he uses the Bamboo Copter to investigate the forest at Mt. Ura. There, Nobita follows another shooting star through the unrolled Roll-Up Fishing Hole and finds Lilulu building a massive robot army. Doraemon, having been suspicious of Nobita’s peculiar behavior at home, follows him there. They enter the mirror world and see a massive base being built by robots. The duo then use a long-range Paper Cup Phone to listen in on Lilulu as she orders the robots to work faster. She is revealed to be a human-hating android. When Lilulu discovers them, Nobita and Doraemon escape back to their own world with the portal accidentally destroyed by Zanda Claus when Lilulu pursues them.
While the two believe everything is safe now, they have completely forgotten about the sphere that is still at home, before it suddenly wakes out and bounces around. After Doraemon puts a Translation Jelly on it, the sphere introduces itself as the ‘brain’ of Zanda Claus while alerting him about a giant robot army from the planet Mechatopia that intends to conquer Earth and enslave all humanity. They try to alert the authorities but in vain, as naturally none of them believe Doraemon or Nobita. With only Suneo and Gian believing him and Nobita about the robot army, Doraemon reprograms the sphere until it is no longer a threat.
They re-enter the Mirror World through Shizuka’s Bathtub by using a special pigment but leave Shizuka out of the mission due to the risky nature of the mission. Using Doraemon’s futuristic weapons, they manage to take out the robots at the base, and soon learn that a massive robot army is approaching Earth within two days. Meanwhile, Shizuka comes to know of the Mirror World and enters it. She finds a girl (Lilulu) who is injured and takes her to her home (in the Mirror World). Later Nobita, Doraemon, Gian, and Suneo rejoin Shizuka. Shizuka uses Doraemon’s machine first aid kit to repair Lilulu. While recovering, Lilulu recounts to Shizuka the history of her planet Mechatopia.
Despite everything Nobita and the others do for her, Lilulu escapes and decides to alert the rest of the Robot Army that they are in a Mirror World and not the real Earth. The gang searches for her with Nobita discovering and then trying to stop her but Lilulu knocks out Nobita with a laser beam from her finger.
Nobita is found by Doraemon and they tail Lilulu to where she meets up with the commander and the officers of the robot army. Surprisingly, she does not reveal the true nature of the Mirror World and even openly questions the Robot Army’s purpose for enslaving humanity, which results in them ordering her arrest. Luckily, Nobita and Doraemon rescue her. Back in Shuzuka’s house in the Mirror World, Lilulu still feels conflicted with her loyalties and willingly allows Doraemon to lock her up in a birdcage using the Small Flashlight.
In the meantime, the robot army becomes suspicious because of the lack of humans in the world. They soon discover that they are in a fake world after analyzing the satellite image of the world and comparing the image to another image of the current world, and seeing how they are reversed. They return to the lake where they had first entered the fake world, which they believe is the connection doorway. Doraemon and the group intercept the army at the lake.
Lilulu and Shizuka remain at Shizuka’s house to have a talk, during which an outburst from Suneo’s robot (who Doraemon had made sentient halfway through the film and joined the gang on their quest) gives Shizuka a brilliant idea to save the world. She re-enlarges Lilulu with the Enlarging Flashlight, and they use the Walk-In Mirror to return to Nobita’s room in the real world, where, using the time machine located inside Nobita’s desk, they travel to 30,000 years ago on Mechatopia, where they talk to the professor who created the robots from which the robot army is descended from. Appalled by what the robot army has become, the professor decides to redo everything by removing the competition instinct from his robots, replacing those instincts with ones of humanity and love. He collapses before he can finish his job. Lilulu decides to complete the salvation, despite being aware that she will disappear after they alter history, and she continues the reprogramming with instructions from the professor.
Back on Earth in the present time, the robot army, much superior in number, has taken the upper hand with Zanda Claus heavily overwhelmed when suddenly, the robot army began disappearing, meaning that the reprogramming in the past is successfully completed, and the robot army is completely erased from history. Back at Mechatopia in the past, Lilulu also begins disappearing, and shares a final handshake with Shizuka before she is erased completely. A heartbroken Shizuka then uses the Anywhere Door to return to Earth, rejoining her friends with sorrow about Lilulu’s end.
After returning to the real world, Doraemon finds Nobita being left behind at school by his teacher as punishment for being absent-minded throughout classes. Nobita confesses to Doraemon that he was deep in thought of Lilulu’s fate and they discuss the possibility of her being reborn before Doraemon goes off to the baseball field. Shortly afterwards, Nobita is shocked to see Lilulu fly past him from the school windows. She gives him an acknowledgeable wave before flying off. Convinced that Lilulu has been truly reborn, Nobita rushes off to tell his friends of the good news while Lilulu watches the Earth from outer space.