The film opens with a lighthearted dispute over what kind of camping trip to undertake: Nobita Nobi, Shizuka Minamoto, and Doraemon want the coast, while Gian and Suneo Honegawa push for the mountains. Nobita’s mom interrupts, insisting he finish his homework, but when news reports the Bermuda Triangle, Gian and Suneo decide to join the others. With Nobita finally finishing his homework, the group sets out for their camping trip.
They travel equipped with an Adaptation Light that lets them survive underwater and a quirky underwater buggy named Buggy that zips them around the sea. Buggy isn’t just a machine; he exhibits feelings and develops a soft spot for Shizuka, drawn to her because she is female. Doraemon pulls out a massive tent that somehow provides everything they need, and that night they settle in for a barbecue. Gian and Suneo want to explore the Atlantic, but Doraemon won’t allow it. The two companions sneak away anyway, taking Buggy with them in hopes of finding a treasure ship, while the rest stay behind.
As they drift into deeper water, the eyes of adventure close around them: Gian and Suneo lose consciousness, the result of the fading effects of the Adaptation Light, and their friends search for them in growing panic. When they finally wake, they discover a news report about a treasure ship that confirms their fears. Meanwhile, Nobita sees a strange fish-shaped craft attack him with an energy weapon and narrowly escapes.
Buggy awakens Shizuka with a vivid memory-like display, showing a memory projector-like sequence of events: a pair of humanoid “sea people” rescue Gian and Suneo using their own Adaptation Light. Soon, the footage confirms the existence of a mysterious underwater craft controlled by two human-like figures. The group then glimpses the same sea-people on the surface, only to have the craft Nobita saw come into view and attack them. A giant squid emerges and lashes out, tearing apart their camp, until a strange light stuns the creature and buys them a moment. The sea-people appear again—but they too are knocked out by the dazzling light.
Nobita, Doraemon, and Shizuka regain consciousness in a room where they meet Eru, a resident of Mu, the underwater kingdom. The Mu Prime Minister greets them but refuses to let them leave, fearing humans’ influence. Gian and Suneo are imprisoned because of their reckless behavior, while the others work to rescue them. Eru soon realizes the group’s escape and gives chase. Doraemon’s hat proves incredibly handy, transforming into a subterranean moving room that makes it hard for Eru to track them. But a fish-ship suddenly attacks Eru, and Gian rescues the group, though their escape leads to all of them being arrested and sentenced to death for crossing the border.
In their cell, Eru tries to defend them, but a ship crash interrupts the proceedings. It’s revealed that Atlantis — another underwater realm now controlled by robots — is poised to destroy the Earth with nuclear weapons, mistaking a volcanic eruption for an invasion. After persuading Mu’s Prime Minister, Eru frees the group, and they embark on a journey into Atlantis, which sits in the Bermuda Triangle. With Doraemon’s hat, they bypass a fleet of fish crafts and the barrier into the kingdom and search for the “Devil’s Castle,” where Poseidon, Atlantis’ main computer, supposedly rests. To follow, Shizuka allows herself to be captured by Poseidon’s iron guards and is led to the castle, giving the others a chance to slip in.
Inside the castle, Doraemon’s gadgetry helps them breach the defenses as hordes of iron guards close in. Shizuka is brought before Poseidon, and the group becomes separated in the chaos. Doraemon falls from a high place, while Eru, Nobita, Gian, and Suneo are seized by the guards. Just as Poseidon readies a sacrificial act to halt the volcano, Doraemon intervenes but collapses unconscious. Shizuka’s tears trigger a dramatic intervention: Buggy, somehow present in Doraemon’s pocket, bursts forth and dives into Poseidon, destroying the machine and the surrounding control. With Poseidon destroyed, the iron guards lose their power, and Shizuka and Doraemon guide the others to safety as the volcano erupts and threatens the castle and its weapons.
Returning to Mu, the friends are celebrated as heroes for averting catastrophe. The film closes with Shizuka gazing at one of Buggy’s screws left behind, while the group bids farewell to Eru and heads home, wiser from an adventure that tested trust, courage, and the power of friendship.
