An ancient wizard, his daughter and a four-armed warrior from Genghis Khan’s time invade Atlantis, age Aquababy into a teenage Aquaboy and mentally control him to gain dominance over Atlantis.
A quiet day in Atlantis finds Aquaman and Mera discussing the eventual fate of their son. Aquaman fantasizes that Arthur, Jr. will grow up to be a crime fighter, and future protector of the seas. Mera, however, desires that their son become a great scientist, and eventual ruler of Atlantis. Distracted by their spirited debate, neither notice "Aquababy" wandering off. Outside of Atlantis, a dragon-shaped submarine descends on the domed city. On board is Genghis Khan's greatest scientist, Tamerkhan. Centuries ago, Tamerkhan saw the era of the Mongol empire coming to it's close. He built a great submarine, and departed beneath the waves with his personal bodyguard, the four-armed Mongol warrior, Attucka the Merciless. They turned to piracy to survive. Tamerkhan invented a formula to retard their aging, effectively making them both immortal. In the 17th century, Tamerkhan fathered a daughter, Liat. Tamerkhan then invented a serum that allowed them all to breathe underwater. The aged Tamerkhan has grown weary of roaming the seas. He has come to Atlantis to see his daughter married off, so that the Golden Horde of the Mongol Empire might live on through her children. Finding his way on board Tamerkhan's submarine, Aquaman's infant son is accidentally doused by strange chemicals. The chemicals rapidly age him from infancy to adolescence. Attucka is about to slay the intruder, when Liat declares her intention to marry the young prince. Meanwhile, Aquaman, Mera, and Aqualad are scouring the environs around Atlantis for Arthur, Jr. They find Tamerkhan's submarine, and are met by the teenage Arthur, Jr. & his bride-to-be, Liat. Taken aback by the sudden changes in their son, Aquaman nonetheless invites the Mongols into Atlantis, throwing a huge feast to celebrate his son's betrothal. Outside the city walls, Tamerkhan's submarine is watched over by the Atlanteans. It suddenly roars to life, releasing a magnetic claw that disarms the Atlantean guards.
Tamerkhan's submarine begins cutting into the Atlantean dome. Tamerkhan focuses the beam from his Dragon Talisman on Arthur, Jr. bringing the young prince under his sway. Aquaman is informed of the submarine's assault on Atlantis. Aquaman is about to attack Tamerkhan, when Attucka orders him to stay his hand or watch Mera die. Aqualad leaps into the fray, only to be instantly put down by the mesmerized Arthur, Jr. Tamerkhan leads Aquaman out onto the balcony so that he can bear witness to the destruction of Atlantis. Tamerkhan demands that Aquaman tell the Atlanteans to cease all resistance and recognize Tamerkhan as their new ruler. Aquaman instructs Mera to create a hard water battering ram, using her power to control and manipulate water. He sets off to attack Tamerkhan's submarine. Tamerkhan has come prepared for Mera's powers, dousing her construct with chemicals, hidden in his ring, that dissipate it. The submarine's magnetic claw hand grasps Aquaman. Immobilized, the monarch of Atlantis sees no alternative but to surrender Atlantis to Tamerkhan. Aquaman is imprisoned in an Atlantean dungeon. Mera, surprisingly, switches sides, using her feminine wiles to garner favor with Tamerkhan, offering herself as his wife. To liven up the wedding ceremony, Mera proposes that Aquaman face Attucka in a duel to the death. Aquaman is given a short sword, and the battle begins. With Tamerkhan distracted by the combat, Mera lunges for his Dragon Talisman, hurling it down to Aquaman, and crying out to him to destroy it.
Aquaman destroys the Dragon Talisman, but in doing so, leaves himself open to a terrible blow from Attucka. Dazed, the Sea King is in no shape to defend against Attucka's killing strike. With his wits returned to him, upon the destruction of the Dragon Talisman, Arthur, Jr. tackles Attucka, saving his father's life. Attucka though is too powerful for Arthur, Jr., quickly gaining the upper hand. A recovered Aquaman rejoins the fight. As he brings the Mongol's own axe to his throat, Attucka counters by drawing his two-pronged sword down on Arthur, Jr. Tamerkhan orders Mera to plead with Aquaman to surrender or their son dies. Aquaman does so, but Attucka, who covets Liat for himself, intends to kill Arthur, Jr. anyway. Only Liat's intervention on his behalf spares Arthur, Jr. The young prince is forcibly brought to the altar to marry Liat. Before he can say his vows, though, he reverts back into infancy. Tamerkhan gathers the chemicals needed to re-age Arthur, Jr. back to adolescence. As he is about to inject the infant with the aging formula, Arthur, Jr. grabs Tamerkhan's ring and swallows it. With his chemical deterrent to Mera's powers lost down the gullet of the child, Mera spings into action, creating hard water hammers to break hers, and Aquaman's, chains. Attucka moves in to strike the Sea King down, but Aquaman loops his chains around Attucka's sword, disarming him. The two struggle for several moments, until Aquaman lands a devastating haymaker. Attucka falls backwards, and is impaled on his own double bladed sword. With Attucka dead, Tamerkhan and Liat flee for their submarine. Mera encases the vessel in a cube of hard water. Mera opens a hole in the block to give Aquaman access to the submarine. Boarding it, he and Mera discover that Tamerkhan and Liat have disintegrated into dust. Their great age has finally caught up to them. The dragon submarine is scuttled at the bottom of the Dungeon Depths. All is right once more in Atlantis.