On board his yacht, Adventure, billionaire playboy Peter Dudley is a man unfulfilled. Escaping into the ocean, Dudley seeks out exotic specimens for his aquarium. Spying movement, Dudley casts his net, and catches Queen Mera, of Atlantis. Taken with her great beauty, Dudley ascends to the surface, taking Mera with him. Before he can reach his yacht, Dudley is attacked by Aquaman. The Monarch of Atlantis frees his Queen, then turns to strike Dudley again. Dudley pleads his ignorance, stating that he mistook Mera for a fish. Dudley invites them both back to his yacht. Dudley begins to obsess over Mera. His desire growing for her as great as his building jealousy of Aquaman. Once the Atlantean couple depart, Dudley immediately sets out to become a better version of Aquaman, in the hopes of winning Mera's heart. To this end, he hires one Dr. Hans Ludorf. The doctor warns Dudley that his surgical transformation process may kill him, or that the results may not be quite as desired. Dudley dismisses Ludorf's concerns. At first the operation would seem to be a success. Dudley becomes an exact duplicate of Aquaman. Seconds later, however, his features begin to distort. He continues to change, mutating into an monstrous ogre-like creature, though still bearing a passing resemblance to Aquaman. He has become the Aquabeast. Dudley bursts from the cabin of his yacht, dives overboard, and swims for Atlantis. Locating the fabled underwater city, Dudley conceals himself within an ore cart, headed for Aquaman's palace. Once there, he thunderously reveals his presence, inside the throne room.
Dudley has come for Mera. As he lumbers towards her, Aquaman attacks. Dudley barely feels the punch. A fierce battle ensues. Dudley, as the enormously powerful Aquabeast, deals Aquaman a crushing defeat. Aqualad steps in to protect the Queen, but Dudley contemptuously swats the teen hero aside. Mera uses her ability, to control and manipulate water, to encase Dudley in a block of hard water. Dudley smashes free. As the Atlantean guard enter the throne room, Dudley abducts Mera, and flees Atlantis. His destination is an ominous area of the sea, known only as "The Forgotten Place." Recovered from Dudley's attack, Aquaman and Aqualad return to Dudley's yacht, Adventure. There, Aquaman confirms his suspicion that the Aquabeast is, indeed, Peter Dudley. Prior to leaving Atlantis for Adventure, Aquaman had telepathically ordered a school of Sentinel Fish to form a line, pointing out the direction the Aquabeast took Mera. Following this line now leads Aquaman and Aqualad to "The Forgotten Place". Much farther ahead, Dudley and Mera come under assault from the Demonoids, the android guardians of the "Forgotten Place". Dudley is more than a physical match for the machine horde, but falls to their helmet blasters. Just as Mera is grabbed by a Demonoid, Aquaman and Aqualad arrive to rescue her. They, too, fall, however, to the Demonoids' stun weapons. With the Demonoids' attention on Aquaman and Aqualad, Dudley grabs Mera, and steals deeper into the "Forgotten Place."
On the verge of unconsciousness, Aquaman, more by instinct than intent, issues forth a telepathic distress call. It is answered by Kraken, a giant squid. Kraken attacks the Demonoids, buying Aquaman and Aqualad enough time to recover and escape. Meanwhile, Dudley and Mera have ventured even further into the "Forgotten Place", reaching the precincts of Hammuri. Hammuri, a giant, intelligent, aquatic, alien simian, has spent untold years marooned on Earth, in the "Forgotten Place". Hammuri is quick to place value on Mera, as her harvested brain could power his long dead computer system. Hammuri deems Dudley, however, to be a horrific abomination, one that should be destroyed. Dudley charges Hammuri, grappling with the enormous ape. Extra limbs project from the back of Hammuri's adaptive space suit, and pummel Dudley into the sea floor. Dudley recovers quickly and resumes his assault on Hammuri. Again an extra arm emerges from the suit, this time grabbing Dudley with an unbreakable grip. Hammuri's suit then grows wings. The giant flying ape carries Dudley out over a powerful underwater whirlpool, filling a deep chasm. Before Hammuri can drop him, Dudley crushes the suit's wings. Hammuri and Dudley both plummet into the abyss, and are lost forever. Aquaman and Aqualad finally catch up to Mera, and bring her home.