Aquaman has, in the interest of peace, lent the Crown Jewels of Atlantis to the Metropolis Museum of Art to display to the general public...
This does not go over well with Aquaman's half brother Orm, otherwise know as Ocean Master.
Black Manta, seeing an opportunity against Aquaman, poisons Orm's mind with thoughts of revenge. Taking the Crown Jewels on land is a terrible disrespect to the might of Atlantis. Gathering an army to reclaim the relics an infuriated Orm marches on the city.
And the Justice league is there to meet him head on...