Nobita and Doraemon were finding out the truth about a real dinosaur. Nobita saw something under the ocean when they come across a cave that leads to an underground world full of dino-people.

Unfortunately, Doraemon's machine allowed the fictional monsters to come to the real world and defeated the entire human races, thus turning the future (Nobita's time) into a demon-ruled world.

The series revolves around an earless robotic cat named Doraemon, who travels back in time from the 22nd century to aid a boy named Nobita Nobi.