When Nobita requests Doraemon to take him to a diving spot, the robotic cat uses his gadgets to create an imaginary underwater world for them to explore. However, their adventure takes an unexpected turn when a mermaid named Sophia appears in Nobita's backyard after the illusion fades. Sophia, a princess from an ancient underwater civilization, befriends Nobita, Doraemon, Shizuka, Jyaian, and Suneo, inviting them to her mystical realm. As they journey to the underwater kingdom, they become embroiled in a conflict between the mermaids and a rival tribe that exiled them eons ago. Tasked with aiding Sophia in locating a fabled sword that could restore harmony to the tumultuous seas, Doraemon, Nobita, and their companions must navigate treacherous waters to bring peace to the warring factions.