The film opens with Nobita Nobi poring over a picture book about the story of Adam and Eve, musing that their descendants must endure many hardships because of that original mistake. Doraemon soon reminds him that the summer vacation homework is due, and Nobita's curiosity about his friends' research grows as he discovers everyone is busy with their own tasks. When Nobita tries to glimpse the future to see a completed project, Doraemon unveils the World Creation Set to help him along, and Nobita begins to assemble a brand-new Solar System from “elements of the universe” dust, dutifully recording every step in a diary.
As the work drags on, Nobita gets bored and is chased by Gian for doing homework after Suneo Honegawa ejects him from the scene. In a careless moment, the artificial sun begins to swallow the other planets. Doraemon quickly rewinds time with the set, and when Nobita declares he won’t quit in the middle of the project, the pair get a chance to witness how life on Earth forms—from tiny cells to fish-like organisms.
Doraemon invites Nobita into his own universe, where the goal is to accelerate evolution. To speed things up, Doraemon seizes the ancestor of land vertebrates, a Eusthenopteron, and uses the Evolution-Devolution Beam on it; however, a stray bug finds its way into the rays. That night, Gian seeks to make amends and join the project, but he spots a strange falling star, following it to two humanoid mantis-like creatures who begin to talk. The creatures notice Gian and chase him, but the Time Patrol intervenes, leaving Gian to believe it all was a dream.
The following day brings the sight of the first land-walking fish and the rise of the dinosaurs, filling Doraemon and Nobita with joy. Yet tragedy soon follows when an asteroid strikes Earth, wiping out the non-avian dinosaurs. Nobita is devastated, but Doraemon offers comfort. Gian and Suneo join the adventure again after Doraemon accidentally takes away Suneo’s manga, prompting Nobita to invite Shizuka to complete the group.
Together they step into Nobita’s universe and witness Earth during the Ice Age, helping a group of primitive kids who resemble Nobita, Suneo, and Gian find their way home. Nobita, adopting the name Kami, grants the children a tool and inadvertently seeds early superstition. That day Gian proudly reveals the project to two friends, drawing the attention of the mantis-like humanoids.
The next day, only Doraemon, Nobita, and Shizuka visit the universe, where they glimpse the dawn of civilization—rudimentary agriculture, witch doctors, and human sacrifices. They also meet the descendants of the primitive kids and encounter a mysterious giant two-headed centipede-like monster guarding a mountain. After defeating the creature, Nobita experiences a peculiar pain in his rear caused by a tiny arrow.
Suneo and Gian decide to drop out for a vacation, which angers the others. The duo are kidnapped by the mantis creatures they had glimpsed earlier. The following day, Doraemon and Shizuka wander to other lands while Nobita travels back to Japan and crosses paths with Nobina, an elderly herb merchant who is treated poorly by Sunemaru, a royal doctor who resembles Gian. Nobina rescues and cares for an injured beetle-like creature Nobita befriends, naming it Chunko. When Nobina’s wife discovers Chunko, she drives him away, and Nobina searches for him, guided by a couple through a cave to an exotic kingdom where he is treated well and rewarded with treasure, ending a long spell of poverty. The daughter of Sunemaru vanishes, sparking a frantic search as people fear a malevolent force; the insect-like citizens create an illusory demonic entity to scare the masses, which is later revealed to be a gathering of thousands of insect people. The daughter is eventually found sleeping by a tree.
The trio continues their journey through many cultures, noticing tiny flying creatures and timeless stories of an underworld that echo across ages and lands. The next leg of the journey leads them to Taishō-era Japan, where they follow an air balloon piloted by the contemporary versions of Dekisugi, Nobita, and Shizuka—Dr. Dekimatsu, Mr. President Nobihide, and his secretary Shizuyo—toward a newly discovered giant hole in the South Pole. There they are halted by a god-like man, whom Doraemon reveals to be another gathering of the mantis-like insects. Using a gadget, Doraemon disintegrates the illusion and the crew presses on.
At the hole’s edge they descend into an underground world inhabited by mantis-like humanoids. Nobihide tries to bargain with their leader, who explains a plan to capture the surface world. The group reunites with Gian and Suneo, along with an insect boy and a robot-like Doraemon. The boy explains his quest to understand evolution from sea to land, and Nobita confesses that he has caused the upheaval. The mantis-like beings prepare to assault the airship, but Doraemon intervenes and reveals that Nobita’s imaginative act created the world itself. He then grants the mantis-like beings a World Creation Set of their own, enabling them to build their own world and thereby safeguard the surface.
In the end, Nobita decides that neither the humans nor the mantis-like humanoids need his help any longer and parts ways for good. The end credits reveal that the four central human characters have all contributed to finalizing the project together, cementing a quiet, expansive closure to their extraordinary journey.
