Doraemon Movie 30: Nobita no Ningyo Daikaisen

Wanting to experience an underwater adventure without leaving town, Nobita asks Doraemon for help, and a collection of gadgets turns the neighborhood into an artificial sea. After the illusion is removed, a mysterious girl named Sophia is discovered nearby. She is no ordinary visitor, but a princess from an ancient civilization living beneath the ocean.

Nobita, Doraemon, Shizuka, Gian, and Suneo are invited to Sophia's underwater kingdom, where they learn that her people were driven from their original home and remain threatened by an old enemy. At the center of the conflict is a legendary mermaid sword. When the struggle reaches the kingdom, the friends join Sophia in searching for the weapon that may be able to protect her people and restore peace to the sea.