Doraemon: Nobita and the Castle of the Undersea Devil

The friends argue over where to go for their camping trip, with Nobita, Shizuka, and Doraemon favoring the coast while Gian and Suneo push for the mountains. The disagreement stalls the plan and reveals the group's split preferences. The opening argument sets up the dynamics for the upcoming adventure.

Nobita's mom tells him to finish his homework, delaying the trip. While he works, news about the Bermuda Triangle headlines the TV, prompting Gian and Suneo to join the others. The plan to camp is revived by the promise of adventure.

After Nobita completes his homework, the group heads toward the coast and prepares for an underwater adventure. They activate an adaptation light to survive beneath the waves and travel using an underwater buggy named Buggy. The scene establishes the safety gear and the futuristic transport they'll rely on.

Buggy, the underwater buggy, reveals his feelings for Shizuka, adding a quirky romance angle to the journey. The moment lightens the mood but also raises stakes as the group progresses toward their destination. The revelation remains a running aside throughout the voyage.

That night the group has a barbecue at camp, enjoying the serene setting. Gian and Suneo want to sail to the Atlantic, but Doraemon discourages them from leaving the group. The tension resolves as they stay put for the immediate mission.

Gian and Suneo take Buggy and venture into the Atlantic in search of a treasure ship. The adaptation light's effects fade over time, leaving them unconscious and vulnerable. Their friends back on shore fear the worst and search frantically for them.

Nobita sees a strange fish-shaped craft that attacks him with an energy weapon. He manages to evade the assault and survive the encounter, but the incident stirs skepticism among the others. The odd sight foreshadows the deeper mystery beneath the sea.

Buggy awakens Shizuka and projects memories like a movie, showing sea people saving Gian and Suneo with their own adaptation light. The group watches this footage intently and witnesses a submerged craft controlled by two humanoid figures. They realize there is more going on beneath the waves than they believed.

Nobita, Doraemon, and Shizuka awaken in a room and meet Eru, a resident of Mu, the underwater kingdom. The Prime Minister welcomes them but refuses to let them leave, explaining that Mu does not want humans to know about their existence. The others contemplate their precarious situation.

Gian and Suneo are imprisoned due to their personalities, while Nobita, Doraemon, Shizuka, and Eru work to rescue them. They manage to flee the kingdom, but Eru realizes their escape and gives chase. The escape escalates tensions as Mu pursues the group.

Doraemon uses his hat to create a moving subterranean room, allowing the group to slip away from Eru's pursuit. A momentary escape is interrupted as a fish craft suddenly attacks Eru's ship. The situation tightens as pursuit intensifies.

The sea reveals another undersea kingdom, Atlantis, now controlled by robots and threatening Earth with nuclear weapons after mistaking a volcano eruption for an invasion. After convincing Mu's Prime Minister, the group journeys toward Atlantis, which lies within the Bermuda Triangle. They bypass a fleet of fish crafts and a barrier to reach the kingdom.

Inside Atlantis, the group searches for Poseidon, the main computer at Devil's Castle. Shizuka allows herself to be captured by Poseidon's iron guards so the others can follow, and the team is separated in the ensuing battle. Doraemon falls, and the others are captured by guards.

Poseidon is destroyed when Buggy crashes into it, triggered by Shizuka's tears. The iron guards stop functioning, allowing Shizuka and Doraemon to rescue the others and escape before the erupting volcano destroys the castle and the weapons. The team escapes with the victory and races back toward Mu.

Returning to Mu, the heroes are hailed for preventing catastrophe. The adventure closes with Shizuka admiring Buggy's screw left behind as they bid farewell to Eru and head home.