Aquaman approaches the Atlantean arsenal, intent on taking Serum X, a formula for turning air breathers into water breathers. To his astonishment, his own guards attack him, declaring that Aquaman is no longer the KIng of Atlantis. Aquaman, along with his octopus ally, Topo, disarm the guards. Undeterred, the guards physically assault Aquaman, forcing the former monarch to beat them into unconsciousness. When Aquaman places his fingerprint on the scanner for identification, he receives an electric shock. In his unconscious state, Aquaman recollects the events that brought him to this point. He had turned over his old enemy, the Fisherman, to French authorities and was en route back to Atlantis. While still in the coastal waters of France, Aquaman chanced to overhear a radio broadcast, detailing news of an Atlantean attack on Monaco. Upon investigation, Aquaman discovered that the Atlantean vessels were under the command of his evil half-brother, the Ocean Master. Protected by an electrical field, Ocean Master was impervious to attack by Aquaman's fish allies. Ocean Master offered to call off his fleet if Aquaman turned over a supply of Serum X. It is the Ocean Master's fondest desire to become a water breather, like his hated half-brother, so that he may carve out his own kingdom beneath the waves.
Aquaman awakes to find himself on the floor of his own throne room, with Mera and Vulko hovering over him. Upon the throne sits the new monarch of Atlantis, Karshon, a man unknown to Aquaman. An Atlantean council unanimously voted Aquaman off the throne, due to his absentee status as King. Too many times the former monarch placed his super heroic life before his responsibilities to the Atlantean people. Now, the Atlanteans have elected a new monarch, Karshon, to lead Atlantis to it's rightful place in the world. Aquaman's former chief of security, Jarsen, informs him that Karshon has pardoned Aquaman for his earlier assault on the arsenal, however, he will not be allowed access to Serum X. The Ocean Master is deemed only a problem for Aquaman, not Atlantis. Mera convinces Aquaman that, though he has no legal right to take Serum X, he does have a moral one. Thus, once more, Aquaman strikes again at the arsenal, taking out the guards in a surprise attack. Aquaman overrides the locking mechanism on the door, circumventing the identity scanner. Waiting within the arsenal is Karshon, with a large Great White Shark. The shark rams Aquaman out of the arsenal, and onto the sea floor. Karshon charges with a harpoon, but Aquaman quickly takes him out. The shark makes another pass at Aquaman, but the Sea KIng wedges a chunk of coral into it's jaws, causing it to swim off. Free and clear to take Serum X, Aquaman's conscience gets the better of him. Leaving the Serum X in the arsenal, Aquaman departs Atlantis to confront the Ocean Master.
Ocean Master spies Aquaman approaching his undersea fortress, with a school of electric eels. Before Ocean Master can signal his fleet to attack Monaco, the electric eels generate a static electric field jamming all communication with the surface. Whales then begin hammering away at the Ocean Master's base. Aquaman makes it to the command station to confront his evil half-brother. A brutal fight ensues. When Aquaman begins to gain the upper hand, his brother breaks away, and launches a trio of missiles at Monaco. Unexpectedly, Mera arrives and encases the missiles in a hard water globe, where they harmlessly explode. With Ocean Master turned over to the authorities, Mera explains her sudden presence. While Aquaman did not take the Serum X, it, nonetheless, had vanished. It's theft was blamed on the former King. Mera barely made it out of Atlantis, with their infant son, Arthur, Jr. Had Vulko not stayed behind and assisted them, they would not have made it out. Aquaman decides that he must return to Atlantis, clear his good name, and regain the throne.