Justice League: Throne of Atlantisis a direct-to-video animated superhero film featuring the DC Comics superhero team the Justice League , which is part of the DC Universe Animated Original Movies . The film is loosely based on theThrone of Atlantisstory arc from The New 52 written by Geoff Johns and serves as a stand-alone sequel to 2014'sJustice League: War. In the film, Arthur Curry , a half- Atlantean prince, discovers his heritage and aids the Justice League to prevent a disaster by his half-brother Ocean Master , claim his throne, and subsequently becomes Aquaman.
Jason O'Mara , Christopher Gorham , Sean Astin , Shemar Moore , Steven Blum , George Newbern and Melique Berger reprise their respective roles fromJustice League: War,while Nathan Fillion reprises his role as Green Lantern / Hal Jordan fromGreen Lantern: Emerald Knights,Justice League: DoomandJustice League: The Flashpoint Paradox.
In the Atlantic Ocean, the USSCaliforniasubmarine picks up incoming human-like threats approaching the sub by sonar. The sub is attacked, leaving the entire crew dead by the invading hostiles. At S.T.A.R. Labs ' Justice League headquarters, Cyborg , who has recently received environmental upgrades in an operation that replacing his remaining lung, is given news by Colonel Steve Trevor , the liaison for the Justice League, over the sunken sub. Cyborg decides to check it himself, deciding against Trevor's idea of calling the other superheroes, as he puts it "there is no Justice League", feeling there is truly no unity to them as they have never come to help each other since Darkseid's invasion.
At Mercy Reef in Maine, Arthur Curry , drunk over the recent death of his father, picks a fight with a group of tough guys over them wanting to eat the lobster Arthur was talking to. He defeats them using inhuman strength and when the last man tries to stab him, his knife breaks into pieces against Arthur's chest. After throwing him into the ocean, Arthur leaves, unaware that he is being watched by Mera and Dr. Shin. Cyborg uses a Boom Tube to teleport down to the sunken sub and discovers hand prints on the hull and that several nuclear missiles are missing from the launch bays. He is then attacked by the same hostiles that attacked the sub and suffers damage to his servos through his escape, though manages to take a knife like weapon off one of his attackers when they stab him with it.
Back at their HQ, Cyborg gets help from Flash and Shazam to get Superman , Wonder Woman , and Green Lantern , leaving Batman the last one remaining. Green Lantern decides to head to Gotham City to get Batman, who is currently chasing henchmen of the Scarecrow . Green Lantern captures them for Batman, but Batman is angry as he needed them to head to where Scarecrow was and about his plot against Gotham's City Hall. With the entire team together, they check the raised sub and Cyborg's assault. They realize the enemy is not related to Darkseid but warriors from Earth's origin. Thanks to Diana's knowledge, she reveals these warriors are from Atlantis , beings saved from death after their land sunk into the ocean after being turned into underwater creatures by their kings mystical trident, after seeing the weapon Cyborg was stabbed with, with the theory of war related to the missing missiles. Using Shazam's suggestion, Superman and Batman decide to meet Atlantis-expert Dr. Shin, while the others are tasked in finding Atlantis.
At Atlantis, Prince Orm and Black Manta , the latter of whom secretly accompanied Dr. Shin when he was watching Arthur disguised as a man named David, meet with Orm's mother, Queen Atlanna . They argue over declaring war on the surface world because of the damage during the events of Darkseid's attack, with Atlanna saying that the volcano that killed the Orm's father the king, was due to Darkseids forces while Orm says it was the Justice League's fight with Darkseid that killed him. Orm also says that the humans polluting the Earth will eventually destroy it and Atlantis in the process, which he believes is a reason to attack them first. Atlanna silences Orm with the Trident of Neptune, the king's trident of the Atlantean legend with immeasurable powers, which seem electrically based, that can only be accessed by Atlantean royalty. and asks Mera to bring Arthur, her other son, to Atlantis. Black Manta uses a craft holographically disguised as a submarine to attack Atlantis using the same stolen missiles, framing the surface for their "unprovoked" attack. Arthur answers the door to Dr. Shin, who tries to tell him that his father contacted him before his death to help Arthur. Moments later, Dr. Shin is killed by Atlantean soldiers sent by Black Manta. Arthur is overwhelmed by the attack, being knocked out and blasted out of his collapsing house, until he is saved by Mera, who dispatches the soldiers uses her control over water and its density, and taken underwater. Batman and Superman enter Dr. Shin's home discovering that his work is destroyed. Superman recreates a photograph of Arthur Curry and a letter from his father revealing that his son is half-Atlantean. Batman decides they must find him. The attack on Atlantis leads the citizens and Orm to demand for war and to break the seal on war plans created by the former king in the event of conflict with the surface. Queen Atlanna decides that they might need to reveal themselves after centuries of being hidden and contact the Justice League. Arthur wakes up with gills on his neck in underwater Atlantean ruins with Mera, who explains his situation. Atlanna met Arthur's father and gave birth to him. Due to her being royal, she could not be with him nor Arthur, so she had to leave to be with her king and Orm, though she still loved Arthur greatly and had even been watching Arthur when he was a child. Arthur had once encountered met Atlanna when he was swimming by the shore. Now, Atlanna believes Arthur can help Atlantis be the bridge between the two worlds. Mera then dresses him with the kings royal garb, hidden by Atlanna inside the ruins. Arthur, having difficulty process the situation, removes the armor and crown from his garb, leaving only the orange and green Atlantean bodysuit underneath, and heads to surface with Mera following him. Soon, they are attacked by the Trench . The creature overwhelm Arthur, one of them even biting his hand, making him bleed. The Justice League arrives in time to defeat the Trench. Orm hears from Black Manta that the Trench failed to kill Arthur. Orm and Queen Atlanna argue because Orm wants to start a war as per his late father's wishes and reveals his knowledge of Arthur Curry. Queen Atlanna reveals to him that she knows he attacked Atlantis. With this news, Black Manta confronts the queen, but is overpowered due to her trident. Orm then stabs her from behind, killing her and taking over Atlantis as the new king.
Arthur, Mera, Superman, Wonder Woman, Green Lantern, and Cyborg arrive at Atlantis and learn of Queen Atlanna's death, the Atlantean people being under the impression that a surface dweller killed her. The heroes are then defeated by Orm, who has now become the super villain "Ocean Master" and uses the trident to incapacitate and restrain them inside cacoon like pods. Superman even sheds blood when attacked with the trident. The group is sent to be consumed by the monstrous Dark Trench, while Ocean Master leads Atlantis' army to the surface. Arthur destroys his pod by tapping into the power of the trident and with Superman's help, as well as his telepathic control over sea creatures, they save the rest and defeat the monster. In Metropolis , a massive tidal wave created by Ocean Master's trident is used to conceal Ocean Master's army, which reaches the shores and begins their attack. The military fights back but are powerless until the League arrive. During the fight, Superman saves John Henry Irons , and Wonder Woman saves Lois Lane and Jimmy Olsen . Arthur is attacked by Black Manta, who reveals that he manipulated Ocean Master and plans to kill him when the time is right and take Atlantis for himself. While Black Manta is revealing his plan, Arthur calls a megalodon to attack him, killing Black Manta. Mera and the League attack Ocean Master but are once again defeated and unable to gain an advantage over him due to his Trident's power, as Shazam turned to his normal form, Billy Batson, and Cyborg is electricuted and disabled. Wonder Woman successfully disarms him of the Trident, though she is nearly incapacitated by its energy when she holds it. Superman is further wounded with the trident, being pierced in the chest with it. Arthur becomes the last to face Ocean Master, who has an upper hand during the fight until the end due to the Orm being unable to use the tridents true power on Arthur due to his own status as Atlantean royalty. Despite this, Ocean Master manages to defeat Arthur with the tridents power by not using it directly on it and causing a massive blast of power to incapacitate him. Batman saves Cyborg from dying by electrocuting him with a taser device, and Cyborg reveals that he has video of Ocean Master confessing to killing his mother when they were captured. As Ocean Master is killing Arthur, nearly impaling him on his trident, Cyborg broadcasts Ocean Master's confession all over Metropolis for Atlantis' soldiers to see. Stunned by his treachery, the soldiers do not obey Ocean Master's commands and Arthur uses the distraction to defeat him. He then convinces the soldiers to stand down and declares his desire to bring Atlantis and the surface world together in peace.
Later in Atlantis, Arthur is crowned as king, with the Justice League among the audience celebrating. Batman suggests that with new threats appearing, they need to solidify the team and Cyborg reveals plans for a watchtower . Arthur joins as Aquaman (which is a nickname everyone on the Internet gave him and he hates), and soon heads with now-love interest Mera against the Trenchers outside Atlantis.
In the post-credits, Ocean Master is incarcerated at Belle Reve , yelling at the guards to release him. He is approached by Lex Luthor who has a proposition for him to consider.