Doraemon The Movie: Nobita's Great Battle of the Mermaid King (ドラえもん のび太の人魚大海戦 Doraemon Nobita no Ningyo Daikaisen?), also known as Doraemon the Legend 2010, is a 2010 Japanese film based on the manga with the same name and was released on March 6, 2010.
As part of the celebration of the franchise's 40th anniversary, and the 30th anniversary of the first Doraemon movie, the film was released theatrically in selected cinemas in Singapore on December 9, 2010 in Chinese.
Nobita asks Doraemon to take him somewhere they can dive and as a result the robot-cat uses several gadgets to make it possible for both to able to swim on an imaginary ocean above sea level. Inadvertently a mermaid ends up on Nobita's backyard when they turn off the illusion. Her name is Sophia and she is a princess of an ancient underwater civilization. Doraemon, Nobita, Shizuka, Giant, and Suneo quickly become her friends and are invited to visit her homeland. While heading towards the underwater kingdom they find themselves amid a conflict between the mermaids and an opposing tribe that has driven them away from their home planet thousands of years ago. Doraemon, Nobita, and their friends must then aid Sophia find a legendary sword that might bring peace once again to the seas.