In 1985 Maine, during a fierce storm, lighthouse keeper Thomas Curry comes to the aid of Atlanna (Nicole Kidman), the princess of Atlantis. After rescuing her, he brings her into his home and cares for her injuries. Atlanna, fleeing an undesired arranged marriage, regains consciousness days later, and the two soon develop a deep love, resulting in the birth of their son, Arthur (Jason Momoa), who possesses the unique ability to communicate with sea creatures.
However, duty calls, and Atlanna must return to Atlantis, leaving Arthur in the care of her loyal advisor, Nuidis Vulko. Vulko, who was once a mentor to Arthur, is tasked with training him. Under Vulko's guidance, Arthur evolves into a formidable warrior, but he faces rejection from the Atlanteans for being a half-breed, leading him to abandon Atlantis and forge his own path.
Fast forward to a year after Steppenwolf's invasion, Arthur intervenes when pirates attempt to hijack a Russian Naval nuclear submarine. He bravely brings the submarine to the surface, defeating the pirates and saving the trapped sailors. However, during the clash, Jessie Kane (Michael Beach), the pirate leader, dies, leaving his son David (Yahya Abdul-Mateen II) consumed by thoughts of revenge against Arthur. In a harrowing sequence, Arthur leaves David to face a grim fate as his father sacrifices himself to save him.
David, inspired by his grandfather's legacy as one of the first Navy frogmen, begins plotting an attack on Atlantis. Encouraged by Orm (Patrick Wilson), Arthur's younger half-brother and the newly crowned king of Atlantis, David targets the underwater kingdom as a means to instigate war against the surface dwellers.
Reluctantly, Mera (Amber Heard), the daughter of King Nereus (Dolph Lundgren) of Xebel, seeks Arthur's help to prevent the looming conflict. She escapes to the surface, where she comes to Thomas' aid from an oncoming tidal wave—a warning from Orm that the oceans are prepared for war. Mera urges Arthur to claim his rightful place on the throne of Atlantis, as he is the firstborn of Queen Atlanna.
Paired with Mera, Arthur meets with Vulko, who encourages him to find Atlan's Trident, the legendary artifact that can solidify his claim to the Atlantean throne. Their quest leads them into a deep conflict when Orm's men ambush them, capturing Arthur while Mera and Vulko manage to escape unnoticed. During his captivity, Orm confronts Arthur, expressing disdain over his existence and revealing that Atlanna was executed as punishment for her union with a human. Instead of accepting a life away from Atlantis, Arthur challenges Orm to a duel within a ring of lava.
As the two clash, Mera saves Arthur just in time, and they proceed to search for the trident across treacherous terrains, ultimately leading them to the Sahara Desert where the truth about the trident is revealed. Subsequently, they journey to Sicily, where they narrowly escape David—now suited in a prototype battle armor as Black Manta—and discover Arthur's destiny as a hero.
Upon reaching the ocean, they confront terrifying creatures known as The Trench but manage to persevere and uncover the uncharted sea where Queen Atlanna is revealed to be alive. Arthur proves himself worthy after facing Karathen, the mythical serpent guarding the trident, and he secures the powerful weapon, granting him dominion over the seas.
The tension escalates as Orm's forces prepare for war, but upon gaining the trident, Arthur rallies his own forces, unveiling his true identity as king. As Orm declares himself Ocean Master, a climactic battle unfolds where Arthur, with the help of Karathen and other marine creatures, confronts Orm. Embracing his place as a leader, Arthur chooses mercy over vengeance, allowing Orm to acknowledge the truth of his brotherly bond and accepting his fate after the return of Atlanna.
In a twist during the mid-credits scene, David is saved by Dr. Stephen Shin who, driven by his obsession with finding Atlantis, teams up with him to exact revenge on Arthur.
