The Coast Guard crew were held hostages by armed Pirates. Aquaman manage to save them and subdues the Pirates. After the Pirates are arrested,Aquaman returns to Atlantis to see Garth training. At Xebel,King Ryus was approached by Nereus,Kraken,& Tula,who informs him about Aquaman. He considers him a threat to his kingdom and sent King Shark to attack the park.
Aquaman,Mera,& Garth are sent to stop King Shark's rampage,but Mera gets captured by Hila,who turned out to be her sister. She & King Shark escaped with Kraken's help,but not before Aquaman notice that he was wearing a necklace,which is the same as his. Back at Xebel,Ryus scolds Mera for her betrayal,but gets killed by Kraken,who just wanted Arthur. Back at Atlantis,Arthur confronts his mother,Atlanna,who reveals that after she left him as a baby,She conceived a son,Orm,by Orvax. When He heard that Atlanna's son is part human,Orvax labels him a threat before Atlanna kills him,but Orm fled with his sister from Orvax's previous marriage.
The Atlanteans are approached by Kraken,who reveals himself as Orm,and challenges Arthur to a combat,which he accepts. The two fought eachother,But Arthur gets overpowered. Orm was to deliver the blow,but was stopped by Atlanna,who begs her to take her because of Orvax's murder before Orm kills her. Tula intervened and helped Arthur,Garth,Mera,& Vulko escaped as Orm took the throne,becoming Ocean Master,and declared war on the dry lands. Tula,who is revealed to be Orm's half sister,isn't very comfortable of his methods and encourage Arthur that Ocean Master will not bring death to Atlantis. Garth was given a suit,dubbing himself as Aqualad.
Arthur & the others fought against Ocean Master's army. Mera,Aqualad,& Tula defeats Hila & King Shark as Nereus fled and Aquaman confronts Ocean Master and defeats him. Arthur became King and everyone bows to him.
Mid-Credits:
As King Shark was imprisoned at A.R.G.U.S.,The Agents informs Amanda Waller that the ship went down and the sailors claim that there was a "a Black diver with red eyes".