Four years after becoming king of Atlantis, Arthur Curry has married Mera and has a son, Arthur Jr., while splitting his life between land and sea. Meanwhile, David Kane / Black Manta continues to seek revenge against Arthur for his father’s death, working with marine biologist Stephen Shin to find Atlantean artifacts. When Shin accidentally discovers a cavern in Antarctica, Manta finds a Black Trident that possesses him, its creator promising to give him the power to destroy Arthur and Atlantis. Five months later, Manta breaks into an Orichalcum reserve on Atlantis to steal it and power his Atlantean machines, which are of ancient origin but of unknown design to modern Atlanteans.

Caught midway, they are chased by Atlantean forces, with Mera injured in the ensuing battle. Arthur learns that this usage of Orichalcum, which emits high quantities of greenhouse gases, has not only raised planetary temperatures and caused extreme weather and ocean acidification. It also nearly caused a planetary extinction in the past, when used by an ancient Atlantean kingdom. To learn where Manta is hiding, Arthur breaks his half-brother Orm out of prison. Together, they visit a pirate haven called the Sunken Citadel, where they meet Kingfish to learn of Manta’s whereabouts. The information they obtain leads them to a volcanic island in the South Pacific, where they battle Manta’s forces after navigating flora and fauna mutated by the Orichalcum.