The story of the second season takes place after its debut in the DCEU, in the movie "Justice League: Throne of Atlantis".
After Orm is deposed from the throne, it is Arthur's duty to assume it. However, Arthur has his doubts about his role as monarch. He doesn't know many things, but one of the things he does know is that he is not a king. And after everything he has seen, he doubts whether his place is really at the bottom of the sea.
While Mera tries to convince Arthur/Orin to deal with his destiny, legacy, and responsibility and accept the throne, Arthur faces an attack from a mercenary, Black Manta. The confrontation leads him to a scholar of the legend of Atlantis.
Even under Mera's suspicion, he gains Arthur's trust, then tells him about the legend of the old king of Atlantis who is sleeping, waiting to be awakened. Soon, with his partner Garth, Arthur ventures out to find where the king's remains are located. They face the forces of the creatures of the trench and various other threats until they reach their destination. However, they learn that it's not so easy to escape destiny when Atlan reveals himself to be a different kind of king from the legends and stories, a ruthless, cruel king, a greater threat than the Ocean Master ever was. Now Aquaman must confront Atlan, the Dead King, and show that he is ready to accept his destiny as king of Atlantis.
Episode synopses can be found in the comments.
