Aquaman and Aqualad, responding to a summons from an old friend, arrive at the schooner home of Captain Crane. The old, retired sea captain has discovered an ancient map to the secret undersea kingdom of King Neptune. Aquaman is skeptical regarding the map's authenticity, but, nonetheless agrees to lead the expedition. The coordinates on the map lead them to an undersea mountain. Aquaman summons lantern fish to provide light, while paddle fish begin excavating the mountainside. Within moments, an ancient door is uncovered. Fastening an anchor chain to the door, Aquaman instructs a trio of giant squid to pull the chain, eventually, dislodging the door from the mountainside. Within the mountain lies a fantastic city of gold. Offering Captain Crane the honor of first discovery, Aquaman ushers his old friend through the doorway. Stepping through the portal, Captain Crane is immediately enveloped in a hidden whirlwind. Aquaman and Aqualad rush to rescue the captain but are pushed back by a sudden underwater explosion. As the water calms around them , they are startled to see the commanding figure of King Neptune, God of the Sea, striding towards them. King Neptune bars the way into the golden city. As Aquaman and Aqualad attempt to get past him, King Neptune waves his trident, causing the aquatic flora, all along the mountainside, to grow and entangle the two heroes. Aquaman summons sawfish to cut them loose. Ducking behind a giant swellfish, Aquaman and Aqualad hope to ambush King Neptune as he nears. King Neptune, however, waves his trident once again, transforming the swellfish into a giant winged seahorse, that he then mounts to give chase to Aquaman and Aqualad.
Pursued by King Neptune, Aquaman and Aqualad dive down to the sea floor. Churning up the sand, the duo vanish into the murky waters. King Neptune gives up his pursuit, preceding to the surface. Aquaman and Aqualad double back to the city of gold, only to find it a barren wasteland, littered with the skeletons of prehistoric marine life. Investigating further, the two discover Captain Crane's scuba gear & a plaque with an erupting undersea volcano etched upon it. Aquaman decides to confront King Neptune, regarding the whereabouts of his old friend. On the surface, they find King Neptune running down a ship, ordering it to leave "his" seas forever. When the captain of the vessel balks at such an absurd demand, King Neptune forcibly hurls the ship back to shore atop an enormous tidal wave. Witnessing the awesome spectacle of King Neptune's wrath, a passing destroyer opens fire on the sea god. With a wave of his trident, King Neptune cause huge spikes to erupt from the water, penetrating the destroyer's hull, sinking it. Aquaman summons giant manta rays, & a pod of sperm whales, to carry the ship's crew back to the mainland. Aquaman distracts King Neptune long enough for Aqualad to make a grab for the trident. King Neptune is too fast, though, & uses the power of the trident to transform Aqualad into a horrific human/crustacean hybrid. The monstrous creature that was once Aqualad turns away from King Neptune and lunges at Aquaman.
Blocking the Aqualad monster's massive pincer with a piece of the destroyer's hull, Aquaman dives beneath the waves. The creature gives pursuit, but Aquaman is able to elude it by passing through an opening in a mountain of coral. The next morning, Aquaman answers a summons from the U.S. Navy. The Navy intends to bomb KIng Neptune, to end his tyranny of the seas. Aquaman pleads with them to reconsider as King Neptune is the only one who can reverse Aqulad's transformation, & also reveal what happened to Captain Crane. Aquaman is given until noon to resolve the situation. Otherwise, Operation:Neptune proceeds & the bombs drop. Returning to the sea, Aquaman swims past an underwater mountain emitting smoke. He is reminded of the image on the plaque of the erupting undersea volcano. Aquaman returns to Neptune's kingdom, to discover that the plaque is actually the cover to a hidden vault. Within the vault is an ancient scroll. The scroll tells the story of King Neptune. Once a powerful sorcerer who protected the seas, King Neptune divined, in his dying years, that a great undersea volcano would erupt, in the far future, setting the seas ablaze, & touching off earthquakes that would bring the cities of man to ruin. Desperate to avert the coming catastrophe, King Neptune transferred his sorcerous might into his trident. The map to his hidden kingdom is placed into a bottle, that will be time released to surface when the volcano's eruption draws near. He casts an illusion of a great golden city to lure the map finder inside King Neptune's lair. The first to enter will be magically transformed into King Neptune's likeness, with one overriding thought dominating their mind... "Protect the Seas!" Aquaman realizes that Captain Crane is King Neptune, & that the U.S. Navy is coming to destroy the one man who may be able to prevent the end of the world. Racing against time, Aquaman reaches King Neptune before the Naval bomber, & pleads with him to dive beneath the waves. King Neptune turns a deaf ear to Aquaman's warning, instead summoning the monstrously transformed Aqualad, to remove Aquaman from his presence. In the distance, Aquaman can faintly hear the drone of the approaching bomber.
Running out of time, Aquaman summons a host of sharks, commanding them to hurl the prehistoric marine life fossils, from Neptune's Kingdom, at King Neptune and the nightmarish Aqualad creature. With his foes thus occupied, Aquaman then commands giant squid to blacken the backs of several pods of whales with their ink. Rising up on a mountain of octopi, Aquaman orders the whales to arrange themselves, upon the surface of the sea, so that their bodies spell out the phrase, "DO NOT DROP BOMBS". With Aquaman too close to ground zero, the bomber holds off on delivering it's deadly payload. Then, the undersea volcano erupts spewing fire & molten rock into the sky. King Neptune has cleared the sea of all passing ships, so there are none on the seas to be endangered. A mountainous tidal wave, though, is heading for the shore. Waving his trident, King Neptune spirals the wave higher & higher into the upper atmosphere until it fully dissipates. With the disaster averted, Captain Crane, Aqualad, & the swellfish all return to normal.