Suneo shows a fossil of a dinosaur claw to everyone except Nobita. Nobita Megumi Oohara is furious and declares he will prove there’s a living dinosaur. Doraemon Wasabi Mizuta refuses to help him, so Nobita digs on a hillside and earns punishment from a nearby landlord who forces him to unearth a hole. He finds an egg-shaped stone, uses a time wrap to restore it to its former form, and after warming it, the egg hatches to reveal a Futabasaurus, Piisuke Ryunosuke Kamiki. Instead of immediately sharing Piisuke with the others, Nobita waits for it to grow while making a deal with the others. However, as Piisuke grows too large, Nobita and Doraemon hide him in the nearby lake but attract the attention of a mysterious assailant from the 24th century who wants to purchase Piisuke from Nobita. Worried about the risk of Piisuke being found and overwhelmed by the burden of care, Doraemon and Nobita decide to act.
They transport Piisuke back in time to the Late Cretaceous, but are attacked by the same assailant who had tried to broker a deal. They manage to escape and reunite with their friends, though Nobita’s claim is met with skepticism. When Gian Subaru Kimura and Suneo Tomokazu Seki confront Nobita about his story, he runs into Shizuka Yumi Kakazu, who is initially inclined to doubt him as well. To prove Piisuke’s existence, Nobita brings them to his house and tries to show Piisuke, but when proof fails, they end up watching Piisuke on a television monitor. They realize, to their shock, that Piisuke has actually been carried to the North American shore after the time machine was attacked. With the clock ticking, the group heads toward that distant coast, even though the time machine itself is overloaded and destroyed upon their arrival.
On the North American shore, they locate Piisuke and finally accept Nobita’s claim. Doraemon suggests the group enjoy the beach while he works on a fix for the time machine, but he delivers only a grim update: the machine is broken and must be returned to Nobita’s desk in faraway Japan if they ever want to go home. That night, a massive Tyrannosaurus emerges from the forest, and Doraemon uses his Momotaro Dango to calm it and guide it back into the wild. The group sets out to cross the ancient land bridge of Beringia, meeting a parade of dinosaurs who either aid or hinder their passage, including Brontosaurus and Ornithomimus, and they fend off a pack of Pteranodons that shred their bamboo-powered flycraft.
Their peril escalates when a group of dinosaur hunters from the 24th century—working for a fossil collector named Dollmanstein—appear and offer to return them home in exchange for Piisuke. The kids refuse the deal, so they craft a clever trap, sculpting mud statues of the hunters and placing them in cars as decoys, and escape by raft. The chase intensifies, and the friends are eventually separated, with Gian, Suneo, and Shizuka captured by the hunters while Nobita, Doraemon, and Piisuke plummet over a waterfall. A gadget from Doraemon saves the trio, but Piisuke must be left behind for safety as they slip away and regroup downstream.
They locate the hunters’ camp, where Gian, Suneo, and Shizuka are used as bait for a roaming Tyrannosaurus. The hunters press for Piisuke, and when the huge predator is unleashed, the Time Patrol arrives to intervene, sealing the hunters’ fate. Piisuke is safely returned to his homeland in the Late Cretaceous of Japan, and Nobita and his friends bid him farewell before finally returning to the present time.
