Doraemon the Movie: Nobita and the Castle of the Undersea Devil

Nobita and his friends embark on a summer vacation beneath the Pacific Ocean using Doraemon's underwater vehicle. Jaian and Suneo venture into the Atlantic Ocean searching for treasure, but their protective environment gun loses power, leaving them vulnerable. They are rescued by marine creatures and later captured, discovering they are inhabitants of Mu, an underwater kingdom. It is revealed that the robotic kingdom of Atlantis, mistaking a volcano eruption for an invasion, plans to destroy Earth with nuclear weapons. With the help of a young boy from Mu, Nobita and his friends attempt to stop the robots. Though captured, Doraemon's robotic underwater vehicle sacrifices itself to destroy the controlling computer. Returning to Mu, Doraemon and his friends are celebrated as heroes.