Four years after becoming king of Atlantis,^[a] Arthur Curry has married Mera and had 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 them to power his Atlantean machines that are of ancient origin, but are of unknown design to modern Atlanteans. They are caught midway and 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, but 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, and they visit a pirate haven called the Sunken Citadel, meeting 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 getting themselves through flora and fauna mutated by the Orichalcum. There, Orm comes into contact with the Black Trident, which shows him visions of its provenance. Orm learns the trident was created by Kordax, the brother of King Atlan and ruler of the lost kingdom of Necrus. Kordax was imprisoned with blood magic following a failed attempt to usurp the throne. Realizing the blood of any of Atlan’s descendants could release Kordax; the pair deduce Manta has kidnapped Arthur Jr. The Atlanteans, with Shin’s help, determine that Kordax’ prison and the lost kingdom of Necrus is located in Antarctica.