During their summer vacation, Nobita and his pals embark on an underwater adventure in the Pacific Ocean with Doraemon's futuristic submarine. Their quest for a sunken treasure ship leads them to the depths of the Atlantic Ocean, where they face a crisis when Doraemon's protective device malfunctions. Rescued by marine creatures, they are taken to the underwater kingdom of Mu, where they learn of a looming threat from the robotic rulers of Atlantis. Teaming up with a young Mu resident, Nobita and his friends strive to thwart the robots' destructive plans. Despite facing capture, a brave sacrifice by Doraemon's robotic vehicle ultimately saves the day, earning them the title of heroes upon their return to Mu.