Suneo shows everyone but Nobita a fossil of dinosaur claw. Angry, Nobita claims he will be able to find a living dinosaur. As Doraemon refuses to help him, he digs on a hillside, but instead earns punishment from a landlord nearby who forces him to unearth a hole in the ground. He finds an egg-shaped stone underneath and quickly uses a time wrap to return it to its former form and after warming it, the egg hatches to reveal a Futabasaurus, who is subsequently named Piisuke by Nobita. Instead of immediately showing it to the others, Nobita waits it grow while making a deal with others. As Piisuke grows too large and is in danger of being found, Doraemon and Nobita transport him to 100 million years ago in the Late Cretaceous period. They are attacked by a mysterious assailant who previously tried to make a deal with Nobita to sell Piisuke, though they manage to escape. Left with no proof, Nobita instead shows them Piisuke through a television monitor, but realizes that he and Doraemon had unknowingly transported Piisuke to the North American shore after the time machine was attacked by the assailant. They and the others decide to go there, but the time machine is overloaded and crashes off.
The group land on the North American shore and find Piisuke, but the time machine is broken and must be taken back to Nobita's desk in faraway Japan if they want to go back to the present time. As they travel across the land connecting North America and Asia, they meet with various dinosaur species who either help or hinder their progress, such as Ornithomimosaurus, Apatosaurus, and Tyrannosaurus. At a cliff, they are attacked by a pack of Pteranodon who break their bamboocopters. They are saved by several mysterious assailants, who reveal they are dinosaur hunters working for a fossil collector named Dollmanstein from the 24th century. They offer to return them back home in exchange for selling Piisuke. Refusing the deal, the group set a lure for the hunters while they escape across a river. However, they are eventually spotted and separated with Gian, Suneo, and Shizuka captured by the hunters.
Doraemon and Nobita find the hunters' quarter at the downstream where Gian, Suneo, and Shizuka are used as baits for a Tyrannosaurus. The hunters demand that they hand over Piisuke in exchange for their lives, but the Tyrannosaurus is revealed to be the one they had previously used a Momotaro Dango for, thus is tame enough for the group to use in attacking the hunters. The hunters are subsequently captured and imprisoned by the Future Time Patrols. Piisuke is transported to his homeland, the Late Cretaceous Japan while Nobita and his friends go back to the present day.