Doraemon: Nobita and the Castle of the Undersea Devil

Doraemon: Nobita and the Castle of the Undersea Devil (ドラえもん のび太の海底鬼岩城) is a 1983 Japanese animated science fantasy film and the fourth Doraemon feature film, released on March 12, 1983, in Japan. Directed by Tsutomu Shibayama, the film is loosely based on the Western myth of Atlantis, the mythical continent of Mu and the 1980 chapter "8 Days at the Dragon King's Palace". It is the oldest Doraemon film without a remake version and the last piece of media to utilize the 1979 series' original art style.

Plot
The film opens with the main characters arguing over where to go for their camping trip, with Nobita, Shizuka and Doraemon deciding to go to the coast while Gian and Suneo decide to go to the mountains. The plan is temporarily foiled when Nobita's mom tells him to go finish his homework, but when the Bermuda Triangle is on the news, Gian and Suneo decide the join the other three. The main cast finally go camping after helping Nobita to finish his homework. They use an adaptation light to survive underwater, and an underwater buggy named Buggy, to commute. Buggy has feelings and likes Shizuka due to her being female. Doraemon takes out a big tent that provides all their needs, and that night they have a barbecue, with Gian and Suneo wanting to go the Atlantic but disallowed by Doraemon. That night, Gian and Suneo take Buggy and travel through the Atlantic Ocean, trying to find a treasure ship. However they didn't realize that the effects of the adaptation light can fade over time, they lose consciousness, and their friends, who have been looking for them, think they're dead, but they somehow wake up after them crying. They find the treasure ship mentioned on the news, Nobita, who is outside, sees a strange fish shaped craft that started attacking him with an energy weapon. He managed to evade it, but the others didn't believe his story.

That night, Buggy wakes Shizuka and reveals he saw what happened with Gian and Suneo when they lost consciousness, showing his memory like a movie projector. The footage shows a strange underwater craft that's controlled by two human like figures. Soon, everyone else watches the footage as well, seeing these "sea people" saving Gian and Suneo with their own Adaptation Light, before the craft Nobita saw appears and starts attacking them. Not long after, a giant squid appears and attacks the group, breaking their camp, but a strange light knocks the creature out. The group then sees the "sea people" in front of them, but they too were knocked out by the stunning light.

Nobita, Doraemon, and Shizuka later wakes up in a room where they meet one of the sea man called Eru, revealed to be inhabitants of the underwater kingdom of Mu. The Prime Minister of Mu welcomes the group, but doesn't allow them to leave due to the kingdom not wanting humans to know about their existence. While Gian and Suneo are put in a cell due to their personalities, the group rescues them and flee the kingdom, with Eru realizing their escape and giving chase. Doraemon then uses his hat, which turns into be a subterranean room that can move, causing Eru to lose sight of them. However, the fish craft suddenly attacks Eru, shooting his ship. Gian rushes out with the group to save Eru, but it caused all of them to be arrested and given the death sentence for crossing the border.

While the group is put in a cell, Eru tries to defend them in a trial, which got interrupted by a ship crash landing. It is revealed that another undersea kingdom, Atlantis, now controlled by robots, is about to destroy the Earth with nuclear weapons after mistaking a volcano eruption for an invasion. After convincing the Prime minister, Eru sets the group free and goes on a journey with them into Atlantis, which is in the Bermuda Triangle. After bypassing a fleet of fish craft and the barrier that leads into the kingdom with Doraemon's hat, they went on to search the "Devil's Castle", where Atlantis' main computer, Poseidon resides. But having difficulty looking for it, Shizuka allows herself to be captured by Poseidon's iron guards, so the group can follow it as she's taken to the castle.

Using Doraemon's weapon gadgets, the group enters the castle, facing hordes of iron guards, while Shizuka is brought upon Poseidon, they got separated. Doraemon falls from a high place, while Eru, Nobita, Gian, and Suneo get captured by the iron guards. Just as Poseidon about to sacrifice Shizuka, thinking it will stop the volcano, Doraemon stops it, but falls unconscious shortly. As Shizuka cries over Doraemon, Buggy suddenly appears from Doraemon's pocket due to Shizuka crying, and goes straight into Poseidon, crashing and destroying both of them. With Poseidon destroyed, the iron guards stops working, allowing Shizuka and Doraemon to rescue the others and leave before the erupting volcano destroys the castle. After returning to Mu, Doraemon and his friends are hailed as heroes.

The film ends with Shizuka looking at one of buggy's screws that it left behind, and the group saying farewell to Eru and returning home.

Release
The film was released in Japan on March 12, 1983.