One day, Nobita finds a stray dog drowning in a river while playing by the river side. Feeling sorry for the dog, he decides to take it home by hiding it in a "kennel on the wall" and names it Ichi (originated from "One", which sounds similar to "wan", the sound a dog makes according to the Japanese language), and secretly feeds it and plays with Ichi with his Kendama to find. Soon after, he also adopts a stray cat named Zubu (or wet in Japanese, due to the discovery of it in a rainstorm)... and many other stray cats and dogs. With so many stray cats and dogs, Nobita, along with his friends, decide to send them back in time, 300 million years ago, where there was no other living beings around. After using the Ray of Evolution to allow them to operate a food-making machine, they depart, with a promise to return to the present.
However, when they try to visit them the next day (from their point of view), they encountered a time-space anomaly, sending them crash landing 1,000 years before their original time-destination— to their surprise, They found out the dogs and cats have evolved enough to form a civilized society that rivaled those of Future Society. As the time machine is broken, they try to find help to repair it so they can continue their chrono-journey, only to learn of a bigger issue — a cluster of asteroids are going to Collide with Earth.
Will Nobita finally reunite with Ichi, as he promised? More over, what's with Hachi, a dog who resembles Ichi greatly? The 300 million years of time and space adventure begins.