Mermaid princess Sophia appears in Nobita's yard. Visiting her underwater home, the gang joins a mer-tribe battle and searches for a mythical sword.

Doraemon uses one of his secret gadgets to create a virtual ocean for Nobita to play in, when suddenly, the mermaid princess Sophia, a descendent of the planet Aqua appears. Her people came to earth 5000 years ago, after being persecuted by the ruthless Kaigyozoku (the monstrous fish gang mer-men). At the mermaid palace, Doraemon and his friends learn about the mysterious mermaid legend and the "Mermaid Sword."