As a summer assignment, Nobita has to research the past of his town. Together with Doraemon, they use a gadget called time camera to take photos of the town sights from various time periods. To their great surprise, the last photo they take depicts Japanese folk hero Momotaro pulling a cart laden with treasure.

Later, Nobita's mom encounters a foreigner who shows the kids a small picture of a person dressed as samurai. The picture looks almost like a photo, and the person in the picture looks exactly like Momotaro. The foreigner has inherited this picture from his ancestors, and he has come to Japan in order to learn the truth about its origins.

Nobita begins to think that Momotaro might have been a real person. In order to solve the mystery, he travels to the past of Japan along with Doraemon, Shizuka, Takeshi and Suneo.

(Source: Wikipedia, translated)