During summer vacation, Nobita and his friends cannot agree on where to go camping. To settle the dispute, Doraemon proposes an unconventional solution: an underwater camping trip in the middle of the ocean. Using secret gadgets such as the Underwater Buggy and the Tekio Light (Adaptation Light), the group descends into the depths to explore the seabed and encounter various marine creatures.

Their adventure takes a serious turn when they discover a sunken ship and meet a mysterious young man named El. He is revealed to be a sea dweller, an inhabitant of the advanced underwater civilization known as the Mu Federation. The people of Mu are deeply distrustful of surface dwellers, creating immediate tension. Before trust can be fully established, alarming news arrives: the Devil's Rock Castle, an ancient and feared weapon system from the lost kingdom of Atlantis, has begun to activate. This automated fortress, controlled by robots, poses an existential threat not just to Mu but to the entire surface world. Mistaking natural undersea volcanic activity for an attack, the computer system known as Poseidon prepares to launch nuclear missiles that would render the Earth uninhabitable.

Nobita, Doraemon, Shizuka, Gian, Suneo, and their new friend El must overcome cultural barriers and work together to infiltrate the Devil's Rock Castle. Key characters include the robotic Underwater Buggy, a vehicle with its own personality that forms a particular attachment to Shizuka. The group faces capture and mortal danger as they race to disable the rogue computer system before it triggers global annihilation. The fate of both the sea and the land depends on their courage and trust in one another.