The film opens with a sunken ship, where a shark glides through the dark water and an octopus sits atop its head beside a curious mermaid. The Merman tribe is in search of the Mermaid tribe, and their mission appears to threaten distant worlds as they destroy another planet in their quest. On land, Nobita's mother asks him to run an errand, but he ends up playing with Gian Subaru Kimura instead, a moment that nudges the day into a series of adventures to come. Doraemon Wasabi Mizuta volunteers to do the shopping, and among the items on the list is Dorayaki, his favorite snack, a small detail that adds warmth to the bustling household errands.
Suneo Tomokazu Seki arrives with Palau diving photos, sharing them with Gian and Shizuka Yumi Kakazu as Nobita watches with a mix of envy and curiosity. Nobita hopes to visit Suneo's house to see more pictures, but he is unwelcome, and Doraemon tries to cushion the sting with a comforting presence. When Nobita voices a desire to dive, Doraemon demonstrates an underwater pump simulator that pulls water out of Tokyo, forcing the city to briefly slip beneath the waves. That night, the duo tests a diving mask simulator, preparing themselves for a deeper look beneath the surface.
Nobita spots a fish tail and insists he’s seen a mermaid, a claim that triggers a cascade of uncanny sightings. They snap photos and catch glimpses of shadows—one of a mermaid and one of a shark—before Nobita’s father reports seeing a shark near a cigarette store. The mystery deepens as they wonder how the creature could be seen without goggles, and they discover the water around them has shifted in strange ways. Dekisugi Shihoko Hagino looks through his telescope and also spots a mermaid and a shark, adding another layer to the growing legend.
Back in Nobita’s world, the mystery intensifies when a girl named Sophia appears, drinking juice and speaking in an unknown tongue. Doraemon tries to translate with a jelly-like device, and the others realize Sophia is not simply a stranger but a being with a mermaid tail rather than legs. Doraemon introduces Sophia to the others, and Gian and Suneo are captivated by her presence. Shizuka, ever curious, agrees to help Sophia, and Sophia allows her to try on a tiara, revealing it to be a royal mermaid symbol. Nobita asks if mermaids truly exist, recounting the moment when Sophia first appeared with a tail instead of legs.
Doraemon’s sister, Dorami Chiaki Fujimoto, arrives through a circle and asks Nobita to draw a mermaid; his sketch makes everyone laugh, yet Dorami uses the moment to explain apparitions, legends, and the misidentifications that often accompany mermaid myths. In Dorami’s bedroom, a scream is heard, and the group rushes to the bathroom to find Shizuka transformed into a mermaid while Sophia remains underwater. Doraemon uses his light and a watertight rope to help them breathe underwater, and they slip into new, shimmering forms as mermaids, with Sophia hinting at greater powers at play—powers that Doraemon compares to those of the goddess Manatia. The group wonders aloud whether the legendary mermaid sword exists.
Doraemon consults a scroll by using his ink, discovering that five stars would form a pentagon—a clue that propels them toward Sophia’s underwater palace. The next day, Nobita grows tired and sinks, but a giant eel appears and the others hide in a cave until they are rescued by a puffer fish and by Sophia’s soldiers, who initially accuse Doraemon, Nobita, Gian, and Suneo of kidnapping Sophia. After a tense moment, Sophia pleads for calm, and the puffer fish are instructed to release them. They make their way to the mermaid palace, where Queen Undine rules the mermaid tribe and Sophia’s grandmother, and Sophia discovers that she has been summoned to assume her rightful role as queen.
Meanwhile, Nobita realizes Shizuka has been left behind. Gian and Suneo return to search for her, but they cannot locate her, and the group fears she has fallen into the clutches of the Merman tribe. The King of the Merman tribe arrives with a demand: exchange the mermaid sword for Shizuka, who has been mistaken for the mermaid princess after wearing the royal tiara. Doraemon crafts a convincing counterfeit with Nobita’s inflated toy sword and presents it to the Merman forces, securing Shizuka’s release. Yet the deception shocks the King when he realizes the sword is fake, and his troops unleash a furious attack on the mermaid castle.
Sophia prays to the mermaid god, who reveals the location of the real mermaid sword. But the Merman king steals the blade and uses its power to unleash an underwater tornado that threatens them all. Doraemon rides a watertight ring and uses a surfing board to spray the king with a special gas, revealing the truth behind the “water” and allowing the group to navigate the chaos. Doraemon lures the king onto an island, where they learn the mermaid sword cannot control the simulated water and cannot harm them. In a climactic duel, the king breaks Sophia’s sword, and Doraemon invokes the Sword Round Lightning to defeat him. With the mermaid sword recovered and Sophia’s cause secure, the ocean begins to purify, and Sophia affirms the true power of her people as the waters return to balance.
In the end, Doraemon, Nobita, Gian, Suneo, and Shizuka return home, carrying with them the memories of an adventure that bridged two worlds and tested their friendships, courage, and belief in legends. The journey leaves them with a renewed sense of wonder and a deeper respect for the delicate harmony that exists between land and sea.
