Nobita stumbles into a mermaid princess's world and suddenly everyone's got a reason to care about him. The movie trades the usual neighborhood squabbles for something genuinely generous: a story where the ordinary kid and his magic cat earn real friendship from royalty, not because they saved the day, but because they showed up. It's Doraemon at its most winning, where the gadgets are just permission slips for the heart.
Nobita wants to experience diving without leaving his neighborhood, so Doraemon creates an impossible ocean that floats above ground. When the illusion collapses, a real mermaid named Sophia washes into their backyard. She's a princess from an ancient undersea civilization, and soon Nobita's entire friend group gets swept into her world. The underwater kingdom isn't the peaceful paradise they expected. A rival tribe has waged war against Sophia's people for thousands of years, forcing them into exile. Now that Nobita and his friends are here, they become entangled in the conflict whether they want to or not. The group embarks on a quest to locate a legendary sword capable of ending the bloodshed once and for all. As they journey deeper into unknown waters, they discover that friendship between surface dwellers and mermaids might be the most powerful weapon of all. It's a typical Doraemon adventure where gadgets create chaos, kids stumble into danger, and somehow they manage to save the day while learning something about courage and loyalty along the way.