Aquaman is turned into a giant when a growth serum created by his friend Professor Brant accidentally splashes on him. Brant is killed by members of a gang headed by the Fisherman, an underwater bandit. The Fisherman steals the serum and uses it to menace Atlantis, which he hopes to loot. Aquaman fights off the Fisherman, who also makes himself a giant for a time. Both Aquaman and the Fisherman are later restored to their normal sizes.
Professor Brant, an old friend of Aquaman's, requests the Sea King meet him, alone, at his private marine research station. Professor Brant demonstrates his latest invention, a formula that induces rapid growth in living things. Though his formula could be of great benefit to mankind, Brant fears it will fall into the wrong hands. As if on cue, a trio of frogmen emerge through the sea lock that feeds Brant's pool. They forcibly take the growth formula from Brant, spilling some of it on Aquaman as they subdue him. Aquaman recovers to find Professor Brant dead. Seconds later, Aquaman begins to grow, crashing through the roof of the research station, before attaining maximum height. Unable to turn up an antidote to the growth formula in Brant's laboratory, Aquaman returns to the sea. In his wake, Aquaman accidentally maroons a passing freighter upon the shoals. At his great size, Aquaman is easily able to lift the freighter, and place it into deeper water. Meanwhile, a strange submersible vessel descends on Atlantis. It's even stranger captain is called "The Fisherman". It was his men that stole the growth formula. Now, that formula is used to enlarge a jellyfish to epic proportions. At the behest of the Fisherman, the jellyfish approaches Atlantis.
The giant jellyfish grasps Atlantis in it's tentacles, then proceeds to pull the domed city loose from it's foundations. Aquaman arrives, and begins to pull the jellyfish off of Atlantis. However, in grabbing the jellyfish, Aquaman suffers several poisonous stings. He is forced to race to a nearby kelp bed, which contains medicinal properties, to treat his wounds. Fashioning a lasso out of the kelp, Aquaman ropes the jellyfish, removing it as a threat to Atlantis. The Fisherman is not pleased that his bungling minions accidentally enlarged Aquaman, during the theft of the growth formula. He sprays the formula on the coral Aquaman is standing upon. Aquaman is engulfed by the rapidly growing mass of tiny organisms. He summons a school of sawfish to cut him loose. Aquaman lifts the city of Atlantis up off the sea floor, and carries it away to a safer location, until he can deal with the Fisherman. The Fisherman releases a scavenger claw from his submersible, in an effort to draw Atlantis back up, out of the crevice Aquaman left it in. Aqualad races from Atlantis to alert the giant-sized Sea King to the Fisherman's newest attempt to steal Atlantis. Aquaman returns, and grabs the scavenger claw, violently shaking the submersible. The Fisherman uncouples the scavenger claw from his sub. Dousing himself with the formula, the Fisherman exits the sub to face Aquaman.
The Fisherman hooks Aquaman with his specialized rod and reel. The Sea King leaps and dives across the ocean floor.He bursts through the surface, then dives back down, ever deeper beneath the waves. Aquaman scrapes the shiny, ensnaring line against razor-sharp rock outcroppings, but still he cannot free himself from the Fisherman's line. Exhausted, Aquaman gives in to the Fisherman's inexorable pull. Suddenly, the line goes slack, and as the Fisherman reels in the last of his line, he finds his hook empty. Aquaman has escaped him. Unbeknownst to the Fisherman, the growth effect had reversed to the extreme, shrinking Aquaman to mite-sized. The still gigantic Fisherman begins angling for Atlantis, easily hooking the great domed city. At his tiny size, Aquaman is uncertain that the marine life can pick up his telepathic commands. Nonetheless, he attempts to get a message through to a passing pilot fish. Soon, a swarm of electric eels descend on the Fisherman's line, sending electric current back up it's length, shocking the Fisherman. The electric shock cancels out the effect of the growth formula, returning the Fisherman to normal size. The Fisherman returns to his submersible, and makes plans to destroy Atlantis with depth charges. Aquaman gains entry to the submersible through the cooling system's intake vent. Despite his tiny size, Aquaman is able to pull a lever which releases the depth charges from the firing rack. The Fisherman attempt to spear Aquaman with a trident. Suddenly, the shrinking effect wears off. With one punch, Aquaman knocks the Fisherman unconscious. Turning the criminals over to marine authorities, Aquaman returns to Atlantis, and the waiting arms of his wife, Mera.