Doraemon: Nobita’s Great Battle of the Mermaid King

A sunken ship lies on the seabed as a shark swims nearby. An octopus lands on the ship's head beside a mermaid, hinting at a hidden world of mermaids and mermen. These visuals foreshadow the larger conflict between mermaid and merman tribes.

In a distant space, the Merman tribe destroys another planet as they search for the Mermaid tribe. They believe mermaids possess power or artifacts that threaten their rule. This sets the stage for a galactic-scale conflict woven into the story.

Nobita's mother sends him to go shopping, but he ends up playing with Gian. She asks Doraemon and others to join, but they decline. Doraemon heads to the shop, drawn to the Dorayaki on the list.

Suneo shows diving photos from Palau to Gian and Shizuka, tempting Nobita with underwater adventures. Nobita wants to see more pictures at Suneo's home but finds himself unwelcome. The scene sets Nobita's yearning for exploration.

Doraemon uses an underwater pump simulator to drain water from Tokyo, causing parts of the city to submerge. That night, Nobita and Doraemon practise diving underwater using a mask simulator. The stage is set for real underwater exploration.

Nobita spots a fish tail and meets Sophia, a mermaid who speaks in an unknown language. Doraemon uses a translation jelly to communicate, and Sophia introduces herself. The encounter marks the first bridge between Nobita's world and the mermaid realm.

A car nearly hits Sophia as she tries to take off, but Doraemon saves her. The sun drains her energy and she faints; Doraemon uses the Anywhere Door to bring her to Shizuka's bathroom. They also test how they might breathe underwater using a light-based method.

Sophia shows Shizuka a tiara—the royal mermaid symbol. Nobita wonders if mermaids truly exist, prompting discussion about legends and real creatures. Doraemon’s sister Dorami later joins to explain apparitions and myths surrounding mermaids.

In Dorami's bedroom, a scream is heard, and when they reach the bathroom, Shizuka is a mermaid and Sophia is underwater. Doraemon uses a special light to help them breathe underwater, enabling them to become mermaids. Dorami clarifies that some legends hold real power behind the tales.

Doraemon reads a mystery ink scroll that hints five stars form a pentagon, and they dive toward Sophia's palace. Exhausted, Nobita and friends are swept along by an encounter with a giant eel, later rescued by a puffer fish and Sophia's soldiers who mistake them for kidnappers. They are placed in a fish tank inside the mermaid palace.

The King of the Merman tribe arrives and demands the mermaid sword in exchange for Shizuka. The mermaid queen Undine summons Sophia, and Shizuka is mistaken for the mermaid princess due to her royal tiara. The group realizes the gravity of the diplomatic incident.

Doraemon crafts a fake sword from Nobita's inflatable toy to stall the aggressors. When the king discovers the sword is fake, his troops attack the mermaid castle. Sophia prays to the mermaid god, who reveals the true location of the mermaid sword and intensifies the conflict as the underwater tornado begins.

Doraemon lures the merman king onto a dry island and locates the others using a watertight ring. He adopts a surfing board to spray the king with a special gas, restoring his visibility of the water. Sophia retrieves the mermaid sword and, in a final duel, Doraemon activates Sword Round Lightning to defeat the king.

With the mermaid sword, Sophia purifies the ocean, demonstrating the true power of the mermaid people. The balance of sea and land is restored, and the heroes prepare to return home.

The team returns home, bringing Shizuka back safely and ending the mermaid-kingdom threat for now. The city and seas are back to normal, and Nobita's adventure becomes a lasting memory.