Upon Nobita's request, Doraemon conjures up an imaginary ocean for them to dive in using his magical gadgets. However, when they turn off the simulation, they stumble upon a mermaid named Sophia in Nobita's backyard, who turns out to be a princess from an ancient underwater civilization. Eager to learn more about her world, Doraemon, Nobita, Shizuka, Jyaian, and Suneo befriend Sophia, who graciously invites them to visit her underwater kingdom. Yet, as they embark on their journey, they find themselves in the midst of a long-standing conflict between Sophia's tribe and an opposing group who banished them from their home planet centuries ago. In order to restore peace to the tumultuous waters, Sophia calls upon Doraemon, Nobita, and their companions to aid her in locating a fabled sword.