The story follows Nobita and his friends during a summer vacation that takes them beneath the Pacific Ocean. While exploring, Gian and Suneo use Doraemon’s underwater vehicle to travel across the Atlantic Ocean in search of a treasure ship. They soon discover that the environmental gun Doraemon had been using to protect them is running low on energy, leaving the group vulnerable to the sea. Fortunately, a number of marine creatures rescue them. However, Doraemon and his friends are later captured by these creatures, who are revealed to be inhabitants of the underwater kingdom of Mu. It is then revealed that another undersea kingdom, Atlantis, has fallen under the control of robots and is on the brink of launching nuclear weapons against Earth after mistaking a volcano.