Doraemon: Nobita and the Castle of the Undersea Devil

Nobita and his friends spend their summer vacation exploring the Pacific Ocean with Doraemon's help. Jaian and Suneo borrow Doraemon's underwater vehicle and venture into the Atlantic in search of a treasure ship. During their journey, the environment gun protecting them from the sea's dangers begins losing power, putting them at risk. Luckily, they're saved by mysterious marine creatures, who later reveal themselves as inhabitants of the underwater kingdom of Mu. However, Doraemon and his friends are soon captured by these beings. They learn that another undersea civilization, Atlantis, has fallen under robot control. Mistaking a volcanic eruption for an attack, the robots prepare to launch nuclear weapons at the surface world. With the help of a young boy from Mu, Nobita and his friends try to stop the robots but are captured. Just as all hope seems lost, Doraemon's underwater vehicle makes a final sacrifice, destroying the central computer controlling the robots and their weapons. After the crisis is averted, the group returns to Mu, where they are celebrated as heroes.