In 1985 Maine, lighthouse keeper Thomas Curry rescues Atlanna, the queen of the underwater kingdom of Atlantis, during a storm. They fall in love and have a son named Arthur, who has the power to communicate with sea creatures. When Atlantean soldiers are sent to retrieve Atlanna, who fled her arranged marriage in Atlantis, she is forced to abandon her family (to protect them from the Atlanteans) and returns to Atlantis, entrusting her advisor, Nuidis Vulko, to train Arthur. Becoming a skilled warrior, Arthur rejects Atlantis upon learning that Atlanna was executed for loving a human and having a half-breed son.

In the present day, sometime after the death of Steppenwolf,[a] Arthur confronts pirates hijacking a Russian Akula-class submarine. Their leader, Jesse Kane, is killed in a failed attempt to kill Arthur, and his son David vows revenge. Orm Marius, King of Atlantis and Arthur's half-brother, attempts to convince King Nereus of Xebel to help unite Atlantis and attack the surface world for polluting the oceans. Nereus notes that if Orm unites all four kingdoms, he will receive the title of Ocean Master, commander of the most powerful force on the planet. A Russian submarine attacks and is destroyed by Orm, convincing Nereus to join forces to defend the oceans.

Nereus' daughter Mera, betrothed to Orm, refuses to aid them and journeys to the surface to ask Arthur for help. He declines, but Mera earns his trust by saving Thomas from a tsunami summoned by Orm. Arthur reluctantly accompanies Mera to a rendezvous with Vulko, who urges Arthur to find the Trident of Atlan, a magic artifact of Atlantis' first ruler, to reclaim his rightful place as king. They are ambushed by Orm's men, and Arthur is captured.

Arthur is chained and presented to Orm, who blames him and the surface for Atlanna's death. He offers Arthur an opportunity to leave forever, but Arthur challenges him to a duel in a ring of underwater lava. Orm gains the upper hand and nearly kills Arthur before Mera rescues him. Arthur and Mera journey to the fallen Kingdom of the Deserters hidden under the Sahara desert, where the trident was forged, and unlock a holographic message that leads them to Sicily, Italy, where they retrieve the trident's coordinates.

Orm meets David, revealing that the submarine that attacked was the one hijacked by David, hired by Orm to stage an attack from the surface to win Nereus' support. Orm gives David a prototype Atlantean battle suit to kill Arthur, imprisons Vulko, and coerces the Atlantean Kingdom of the Fishermen to pledge their allegiance to him by killing their king after he declines and forcing the queen and princess, who is next in line for the throne, to assemble their forces out of fear.

An armored David rechristens himself as Black Manta and ambushes Arthur and Mera in Sicily, grievously injuring Arthur before he is thrown off a cliff. Mera nurses Arthur's wounds as they seek the trident and encourages him to embrace his destiny as king. At their destination, Arthur and Mera fend off amphibious monsters of the Trench, and a wormhole transports them to an uncharted sea at the center of the Earth. They are reunited with Atlanna, who, 20 years earlier, was sacrificed to the Trench for having Arthur but had survived and escaped to the uncharted sea, alive and well.

Arthur faces the Karathen, a mythical leviathan and keeper of the trident, and voices his determination to protect both Atlantis and the surface, proving his worth and reclaiming the trident, which grants him control over the seven seas. Orm and his allies lead an army against the crustacean forces of the Kingdom of the Brine to complete Orm's preparations. As Orm declares himself Ocean Master, Arthur and Mera, with the assistance of Karathen and the Trench, lead an army of marine creatures against him. Orm's followers embrace Arthur as the true king upon learning he wields Atlan's trident. Arthur defeats Orm in combat but spares his life, and Orm accepts his fate after discovering that Arthur has rescued their mother. Atlanna reunites with Thomas, while Arthur ascends to the throne with Mera by his side.

In a mid-credits scene, David is rescued by Dr. Stephen Shin, a marine scientist and conspiracy theorist obsessed with Atlantis, and agrees to lead Shin there in exchange for help in his revenge on Arthur.
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