Aquaman and Aqualad are visiting Queen Mera in Dimension Aqua, where, in his honor, a colossal statue of Aquaman is being unveiled. A temporal whirlpool forms, disrupting the ceremony, and heralding the arrival of Poseidon, ancient Greek God of the Sea. From the distant past, Poseidon has spied upon Mera. He has now come to claim her as his bride. Mera spurns Poseidon, declaring herself for Aquaman. With a wave of his trident, Poseidon enlarges a seahorse to the size of a suitable mount, then charges Aquaman. Aquaman telepathically commands the seahorse to throw Poseidon. Poseidon uses his trident to cut the seahorse off from Aquaman's power, to bring it more fully under his sway. Poseidon transforms the seahorse into an enormous sea dragon, commanding it to burn Mera's kingdom down, unless she relents, and agrees to marry him. For the sake of her people, Mera acquiesces. Aquaman, though, won't let her go without a fight. He tackles Poseidon, and the two struggle for several moments. Poseidon gains the advantage, and prepares to deliver the death blow. Mera stays his hand, pleading with Poseidon to take her now, back in time, to his kingdom. Poseidon crushes a strange pod from his belt pouch, recreating the temporal whirlpool. With Mera clasped tightly in the sea god's arm, they fade away. Mera's people are outraged at Aquaman's failure to save their Queen. They tear down his statue, demanding he leave Dimension Aqua immediately, never to return.
During his brief struggle with Poseidon, Aquaman palmed one of the pods from the sea god's belt pouch. Aquaman shows the pod to Mera's people, declaring his intention to go after her, and reclaim her from Poseidon. Aquaman crushes the pod, as Poseidon had done. A temporal whirlpool forms about him. Aqualad is just quick enough to leap into the whirlpool, too, before it vanishes. Arriving in ancient Greece, Aquaman and Aquald ascend Mount Olympus. Their request for an audience with the King of The Gods, Zeus, is granted. Zeus is sympathetic to Aquaman's plight. Never one to let an opportunity to humble his brother pass, Zeus arranges a contest between Aquaman & Poseidon, with Mera as the prize. To counter the power of Poseidon's trident, Zeus gives Aquaman an amulet that will protect him from the trident's magic. Zeus places a golden apple within a sealed underwater cave. Whoever can bring it back to him first, unaided, wins the contest. Unbeknownst to Aquaman, Poseidon's man, Proteus, has mystically purloined the protective amulet from Aquaman's person. Aquaman and Poseidon leap from Mount Olympus into the sea. Aquaman, the much faster swimmer, takes an easy lead over the sea god. Poseidon, however, merely uses the magic of his trident to create great iron balls, shackled to Aquaman's wrists. The weight of the heavy metal spheres drives Aquaman to the sea floor. Aquaman summons sawfish to cut through the chains, but they prove unequal to the task. Aquaman telepathically calls out to a pair of jellyfish, instructing them to swim back and forth over his wrists, until they are slimy enough to slip free of his manacles. Aquaman reaches the sealed cave, to find the great iron door swung wide open. Still hoping to find the golden apple before Poseidon can, Aquaman quickly swims into the cave. Poseidon bolts the door shut behind Aquaman, leaving him sealed in the cave forever.
Trapped within the cave, Aquaman is still able to issue out telepathic commands to the surrounding sea life. Aquaman sends a swordfish to spear the golden apple from Poseidon's grasp. Poseidon prepares to pursue the swordfish, when an octopus assaults him, making off with Poseidon's trident. Meanwhile, Aquaman has also sent for a giant manta ray, which, with great effort, manages to tear the iron door from the cave mouth, freeing Aquaman. Aquaman returns to Mount Olympus with both the golden apple and Poseidon's trident, much to Zeus' great glee. Zeus laughs at and taunts the humbled sea god, casually tossing Poseidon's trident back to him. Aquaman, however, states that, in all fairness, he did not win the contest. He broke the rules by calling in the fish to assist him. Zeus decrees that such honesty should be rewarded, naming Aquaman the winner anyway. Outraged, Poseidon summons the North Winds, destroying Zeus' throne room. Abducting Mera, Poseidon flees back to the present. Mera uses her water wielding power to create an enormous, hard water sea creature. Poseidon uses the power of his trident to temporarily erase Mera's powers, but it's too late. The creature is still coming from him. Only now, Mera has no control over it. The creature's first strike shatters Poseidon's trident. Both powerless, Poseidon and Mera swim as fast as they can to escape the hard water construct. Aquaman and Aqualad arrive in the present, just in time to see Mera's creature closing in on her and Poseidon. Spying a makeshift wooden raft adrift on the waves, Aquaman and Aqualad leap aboard. With dolphins powering the raft, Aquaman and Aqualad are just able to rescue Mera and Poseidon, before the hard water creature devours them. Summoning a school of angler fish, Aquaman instructs them to attract the creature with their luminous illicium. The creature turns to pursue the angler fish, which lead it ever deeper beneath the waves, until the intense oceanic pressure crushes the hard water construct. Humbled again for having to be rescued by Aquaman, Poseidon apologizes for his all he's done. Aquaman provides the sea god with a pod, given to him by Zeus, to create a temporal whirlpool. Poseidon returns to ancient Greece, and Mount Olympus.