Doraemon, the gadget-loving robot cat, returns in another movie in this nostalgic anime series. Alongside him are the blunder-prone Nobita, the elegant Shizuka, the jealous Suneo, and the gentle giant Gian. The five friends set off on an undersea camping trip for some fun, not realizing how deep the sea actually is. As they descend further, a new, unknown civilization emerges before their eyes.

They encounter the Mu Federation, an advanced civilization of deep-sea dwellers, where they meet El, a native of Mu. He explains that him people harbor animosity toward surface civilization, rooted in a history that has left the locals fearful of outsiders like Doraemon's gang.

Things take a turn when an automated military fortress threatens both the Mu Federation and the surface world. Poseidon, a rogue supercomputer, controls the Devil's Rock Castle, forcing the gang to set aside their tense relations with the Mu people and team up to defeat it.