Arthur Curry, now married to Mera and a father to young Arthur Jr., navigates a dual existence between the land and sea. He resides in his father Tom's lighthouse while ruling as the King of Atlantis. Despite his royal responsibilities, Arthur finds himself increasingly bored with the political affairs of Atlantis, especially since the Atlantean council vehemently opposes his vision of establishing contact with the surface world.
Meanwhile, a vengeful force emerges in the form of David Kane, also known as Black Manta, who is on a relentless quest to seek revenge against Aquaman. His path leads him to marine scientist Dr. Stephen Shin, as David hunts for ancient Atlantean relics. During his exploration of an Atlantean ruin buried beneath Arctic ice, he discovers a black trident that carries with it a dark promise from its creator: assist in restoring his Atlantean tribe, and as a result, receive the power needed to annihilate Aquaman, his kin, and his kingdom.
Fast forward five months, the consequences of climate change loom large as global warming spirals out of control, wreaking havoc on surface-level ecosystems. The ocean is not spared either, suffering from acidification and disease among its inhabitants. During this turmoil, Atlantis falls under attack by Black Manta, who successfully steals the kingdom’s precious orichalcum reserves to fuel his ancient restored machines with the assistance of Shin. In a harrowing battle, he injures Mera and proves himself to be a formidable opponent for Arthur.
When Arthur finally reconnects with his mother Atlanna and Mera's father Nereus, he learns of David's sinister plans that are linked to the increasing temperatures. This misguided pursuit of orichalcum, a potent greenhouse gas emitter, nearly leads to a planet-wide catastrophe reminiscent of a bygone ancient war. To uncover the whereabouts of Black Manta, Arthur seeks the help of his half-brother Orm Marius, currently imprisoned by the Fisherman kingdom for a grave crime.
Arthur successfully liberates Orm and, after a clandestine meeting with the crime lord Kingfish, they trace David to a remote Pacific volcanic island. There, they encounter bizarre, mutated life forms, a consequence of the ongoing chaos surrounding the orichalcum furnace—Black Manta's hidden lair. In a fight against time, before Atlantis can strike, Orm comes into possession of the black trident and experiences visions of its origin, revealing Kordax, a brother of Atlantis' founder, King Atlan. Kordax, once a ruler of the long-lost kingdom of Necrus, was sealed in ice due to a devastating conflict with Atlantis.
The plot thickens as it becomes clear that the bloodline of Atlan can break the seal that confines Kordax. This chilling revelation propels David to assault Tom Curry's lighthouse, abducting Arthur Jr. and making haste for Necrus's concealed domain beneath the Antarctic ice.
In a fierce confrontation within Necrus, Arthur engages David to protect his son, but finds himself nearly overpowered. Just then, Orm arrives, stripping David of the black trident. As Orm wields the trident, the spirit of Kordax shifts from David to Orm, instigating a brutal showdown between the half-brothers. Following a desperate struggle, Arthur throws the black trident, which Kordax catches but uses against itself to destroy it, resulting in his own demise.
As Necrus begins its collapse, David faces a grim fate. He declines Arthur's offer for rescue and plunges to his death. The Atlanteans, alongside Shin, manage to escape, while Arthur and Mera choose to let Orm go free, fabricating his death tale under the condition that he stays hidden from Atlantis.
Determined to bridge the worlds above and below, Arthur steps into the limelight, disclosing Atlantis's existence to the human world at the United Nations. His aim is to advocate for a united front against the climate crisis and to secure Atlantis's place as a beacon of hope in the global community.
