One day, Nobita picks a small rocket from which a small-sized humanoid alien named Papi comes out. He comes to Earth from a planet name Pirika to escape its evil PCIA army. At first, Doraemon and his friends are confused by Papi's small size, but with the gadget "Small Light", they shrink and play together.

However, the whale-shaped battleship which was chasing Papi comes to Earth and attacks them. Papi blames himself for having involved everyone but, tries to fight against the PCIA army. In order to protect Papi and his planet, Doraemon and his friends travel to Pirika.