Nobita stumbles into a mermaid princess's world and suddenly everyone's got a reason to care about him. The movie trades the usual neighborhood squabbles for something genuinely generous: a story where the ordinary kid and his magic cat earn real friendship from royalty, not because they saved the day, but because they showed up. It's Doraemon at its most winning, where the gadgets are just permission slips for the heart.

Nobita asks Doraemon for a place where they can go diving, so the robot cat uses a handful of gadgets to let the two of them swim through an imaginary ocean that sits above sea level. When they switch the illusion off, a mermaid is accidentally left behind in Nobita's backyard. She is Sophia, a princess of an ancient underwater civilization. Doraemon, Nobita, Shizuka, Gian and Suneo quickly befriend her and are invited to see her homeland. On the way to the undersea kingdom, they are caught up in a conflict between the mermaids and a rival tribe that forced them off their home planet thousands of years ago. The friends must now help Sophia search for a legendary sword said to be able to restore peace to the seas. A comedic Doraemon feature film.