When an Atlantean is menaced by a great two-headed beast, Aquaman rescues the man, and meets a newcomer—Kaltor, an Atlantean who trained him in the use of his powers when he was a teenager. Now Kaltor claims that his fighting strength has waned, and makes Aquaman swear a sacred oath to destroy the two-headed monster. But Kaltor’s daughter Starene arrives, and is sorrowful that her father has made Aquaman swear the oath. Later, the monster kidnaps Mera, but Aquaman refuses to let Aqualad destroy it, opting to capture it instead. As the Sea King had deduced, the two-headed beast soon metamorphoses into Kaltor. The Atlantean tells Aquaman that since being exposed to strange fumes in an underwater mine, he has been transformed periodically into the monster, and fears the next change will be permanent. Kaltor becomes the beast again, and after Aquaman finds Mera, the Sea King steels himself and slays the creature. But the monster transforms back into a living Kaltor, who is reunited with his daughter Starene. Aquaman says that the “death” of the monster was evidently the antidote to Kaltor’s curse.
In celebration of the new monarchs, a new statue of Aquaman and Mera, carved from diamond coral, is unveiled in Atlantis. For his contributions to the Atlantean's continued well-being, Aqualad is given a new mount, a giant sea horse, called "Sea Imp". The festivities come to an abrupt close, however, when word reaches Aquaman that a tremendous, dual-headed creature has made off with an Atlantean farmer. Aquaman and Aqualad immediately set off to rescue the Atlantean. Locating the creature, lurking half out of the mouth of a cavern, Aquaman lures it out, while Aqualad enters the cavern and leads the Atlantean to safety. The creature spies Aqualad, and the Atlantean, escaping it's lair. It turns from it's pursuit of Aquaman to go after them. Spooked by the horrific creature, Aqualad's mount bucks, hurling Aqualad, and the Atlantean, to the sea floor. The creature rapidly closes on them. Aquaman hurls a large, sharp piece of shell, beaning one of the creature's heads. It lashes out, striking Aquaman, and rendering him unconscious. Aqualad races to save his mentor. As he is about to pounce on the creature, it vanishes. As Aquaman recovers, he and Aqualad are greeted by Kaltor, Aquaman's Atlantean mentor. Kaltor was the first Atlantean Aquaman ever encountered. He taught Aquaman many, many vital lessons in how best to live beneath the waves. Kaltor is hunting the creature. He is old, though, and fears he does not have the strength to long continue. He has come seeking Aquaman's aid in destroying the creature. He asks Aquaman to return with him, to Atlantis, where Aquaman can swear a solemn oath, before the Atlantean High Council, to slay the creature. En route, Aquaman and Aqualad are ambushed by the giant Atlantean villain, Lukhan.
Kaltor knocks Lukhan out with one blow, then frees Aquaman and Aqualad. Returning to Atlantis, they are met by Queen Mera, and Kaltor's daughter, Starene. Aquaman, as promised, takes the oath. Aquaman, Aqulad, Kaltor, and Starene depart to hunt for the creature. Unbeknownst to the quartet of adventurers, Mera also follows, spurred on by her jealousy over Starene's attentions to Aquaman. Kaltor splits off from the group to search the badlands. The creature attacks Mera, destroying her chariot, then dragging her away, deep into a labyrinthian series of caverns. Finding a conch horn, Mera blows a distress signal. Aquaman, Aqualad, and Starene, hearing the sound, investigate, and spy the creature burying Mera's chariot. They pursue the creature into the cave. Aqualad breaks off a stalactite, hurling it down on the creature. Aquaman lassos the stalactite, preventing it from impaling the creature. He then lassos one of the creature's heads, instructing Aqualad to similarly tie up the other head. Leashed, the creature flees deeper into the caverns. Aquaman feels the leash suddenly go slack. As he and Aqualad round the corner, they see the creature in the final stages of it's transformation into Kaltor. Through a series of observations, Aquaman had already guessed the truth about the creature, which was the reason he stopped Aqualad from killing it. Kaltor explains that exposure to a strange gas, while rescuing Atlantean miners, was the cause of his affliction. As Kaltor begins to transform once again, this time permanently into the creature, he pleads with Aquaman to end his torment, and slay him.
The creature lashes out at Starene, as it darts down another cavern. Aqualad tends to Starene, as Aquaman summons eels to flood the caverns, seeking out both the creature and Mera. The eels lead them to Mera. Then, the creature attacks. Mera's cavern chamber is littered with treasure. Aquaman and Aqualad are able to procure weapons from the piles of lost currency. Aqualad harpoons one of the creatures heads, but, having swum too close to the leviathan, is snatched up in one it's great claws. Aquaman attacks with a sword, slashing away at the creature's other head, with greater and greater force, until the sword breaks. With the loss of his weapon, Aquaman ushers Mera and Starene deeper into the caverns, to momentarily escape the creature. Using the diamond from Mera's wedding ring, Aquaman carves a saber from a large sea shell shard. Aquaman resumes the attack. The creature grabs Aquaman, and begins to crush the life out of him. With his last gasp, Aquaman musters up the strength for one last strike, plunging the saber deep into the neck of the creature, slaying it. Upon it's death, the creature transforms back into Kaltor, who, remarkably, is still alive. Slaying the creature has reversed the effects of the gas on Kaltor. All return to Atlantis for a celebratory feast, where Aqualad laughably proves that he still hasn't quite tamed his new mount, Sea Imp.