Old Ichi [Osamu Saka] is introduced as an elderly dog who steps into a time machine, hoping to meet the person who once gave him a kendama. A sudden time-space anomaly disrupts the device and sends him spiraling back to infancy, where a researcher cat discovers the newborn pup and begins to care for him. In the present, Nobita [Noriko Ohara] comes across a stray dog struggling in a river after a playful outing with Gian [Kazuya Tatekabe] and Suneo [Kaneta Kimotsuki]. Touched by the sight, he brings the dog home in secret, naming him Ichi. Soon after, he also takes in a stray cat he discovers in a rainstorm, Zubu [Yuko Mizutani], and the two unusual pets become part of Nobita’s everyday life, stirring his mother’s suspicions.
To keep their new companions hidden, Nobita and Doraemon [Nobuyo Oyama] slip away with the Anywhere Door, retreating to the mountains where they uncover a larger problem: a growing number of abandoned animals threatened by deforestation. Faced with responsibility, Nobita and his friends hatch a bold plan. They decide to send the animals 300 million years into the past—before Earth harbored complex life—using the Ray of Evolution. The animals learn to use a food-making machine, allowing them to sustain themselves in their new era, and Nobita makes a promise to Ichi that he will return someday.
This dream is interrupted the very next day when a time-space anomaly yanks them a millennium into the future. The plan backfires: the dogs and cats have, through rapid evolution, built a sophisticated, futuristic society. With their time machine damaged, Nobita’s group must adapt to this unexpected world while they search for repairs. Along the way, they encounter a gang of teen thieves—Bulltaro [Hisao Egawa], Duk [Tomokazu Seki], Chiko [Hitomi Shimatani], and their self-appointed leader Hachi [Megumi Hayashibara]—and Nobita grows suspicious that Hachi might be Ichi, despite the centuries that separate them.
Driven by a sense of solidarity, the kids decide to infiltrate an amusement park—believed by the thieves to be concealing their imprisoned parents—and drill into a restricted room where a time machine lies. Their plan is thwarted when guards overwhelm them and Doraemon [Nobuyo Oyama] is captured, with the rest of the group imprisoned as well.
Meanwhile, a looming threat from space alarms the government: a dense cluster of asteroids is headed for Earth. They requisition Noradium, a rare material essential for evacuation spacecraft, only to have it stolen by Nekojara [Shigeru Izumiya], a corrupt official with his own agenda. Doraemon awakens to Nekojara’s reveal: he intends to use a Noradium-powered time machine to exact revenge on humans for abandoning animals. He plans to manipulate the Ray of Evolution’s devolve function, a move foreseen in a prophecy authored by his ancestor, Zubu [Yuuko Mizutani]. Nekojara exploits a ruse to force Doraemon into fixing the ray, threatening Shami [Mika Kanai], a beloved cat idol who serves as his minion.
Freed from captivity, Nobita and his allies launch a daring mission to rescue Doraemon and reclaim the Noradium. Doraemon breaks free, and Shami turns against Nekojara after a direct confrontation. Nobita and Hachi work together to stop the time machine, but a meteor strikes the device, sinking it and destroying the Noradium in the process. Hachi is swept underwater, and Nobita dives after him to pull him to safety. The rescue triggers a startling memory: Hachi is actually Ichi from the future, the elderly dog Nobita met long ago. He recalls a Nobita-shaped statue made of pure Noradium that he had hidden, and the team retrieves it to present to the government. Nekojara makes a fresh bid for power, but his assault is thwarted.
With the Noradium secured, the government completes a fleet of evacuation spacecraft capable of rescuing everyone, including Nekojara and his minions, just before the asteroid impact devastates the city. The film closes with Nobita and his friends saying farewell to Ichi and the other animals as they depart the city, moments before its destruction. In the post-credits epilogue, Nobita and the group return to the present to share their stories and say their goodbyes.
