During summer vacation, Nobita and his friends can’t agree on a camping destination. At Doraemon’s suggestion, they decide to go camping right in the bottom of the ocean!

Using the secret gadgets “Underwater Buggy” and the “Adaptation Ray,” the five friends enjoy an undersea camping trip while encountering all sorts of marine life. After discovering a sunken treasure ship, they meet a mysterious young man named El. To their surprise, he is a “Sea Dweller” living in the vast Mu Federation beneath the waves!

The Sea Dwellers, who harbor a dislike for those from the surface, find it hard to trust Nobita and his friends. In the midst of this tension, urgent news arrives: “Castle of the Undersea Devil… has begun to move!!”

What exactly is this “Castle of the Undersea Devil” that the Sea Dwellers fear so much? With hearts full of trust for their friends, the group sets off on a grand adventure with the fate of the Earth hanging in the balance!