In the vast and treacherous underwater world of the seven seas, Arthur Curry, also known as Aquaman, has taken on his role as the king of Atlantis and the protector of the deep. As tensions between the surface world and the undersea kingdom escalate, Arthur faces challenges to his leadership from both realms. His quest for peace is further complicated by ancient laws and the emergence of a formidable new adversary who seeks to claim the throne for himself, threatening to unleash chaos on both the underwater and surface worlds.

Arthur is forced to confront unresolved issues from his past, including his complicated relationship with his half-brother Orm, who is imprisoned for his previous attempts to wage war against the surface. Meanwhile, a new discovery leads Arthur and his allies on a dangerous journey into uncharted territories in search of the "Lost Kingdom," a mythic place that holds the key to preserving the balance between the surface world and the ocean depths. Along the way, they encounter mysterious creatures and ancient wonders, but they are not the only ones seeking the Lost Kingdom's secrets, setting the stage for a clash that will determine the fate of their world.