Nobita stumbles upon a mysterious underwater civilization buried ten thousand meters beneath the ocean's surface, a realm so isolated and ancient that humanity above has never glimpsed it. When he and his friends venture down into this submerged world using Doraemon's gadgets, they discover a society with its own culture, technology, and secrets. But this discovery comes with danger. An unexpected threat emerges from the depths, one that could endanger both the underwater kingdom and everything above the waves. Nobita must navigate between two worlds, caught between curiosity and responsibility as he realizes that some discoveries demand difficult choices. This film balances wonder with genuine stakes. The underwater setting creates an atmosphere of isolation and discovery, where familiar characters confront challenges that test their friendship and courage. It's designed for viewers who enjoy adventure that moves beyond simple sight-seeing, who want their wonder mixed with real tension, and who appreciate when children's stories refuse easy answers. The contrast between the familiar world and this hidden realm drives the emotional weight of the story.