As always, Suneo's boasting makes Nobita very angry. In order to help Nobita realize his wish of diving, Doraemon turns the whole city into a virtual ocean and uses props to attract creatures in the ocean. A mermaid came to the city, but was trapped by mistake. Her name is Sophia, and she is the princess of the mermaid tribe. It is said that the mermaid tribe lived on another planet five thousand years ago. The peace-loving people came to Earth to seek refuge because of the invasion of another tribe, and have taken root here ever since. Nobita, Shizuka and others soon became good friends with Sophia and promised to help her find the "Mermaid Sword", the legendary treasure of the mermaid tribe. Just as the friends were playing in the deep sea, danger came unexpectedly. The alien demons who invaded the mermaid tribe's hometown also came to Earth to find the mermaid sword. The friends are about to embark on a new adventure... This film is a commemorative work for the 30th anniversary of the launch of the Doraemon theatrical version.