Summoned to a remote seacoast laboratory, Aquaman and Aqualad bear witness to Dr. Deering's latest invention, an immense ocean-going submersible. Following Dr. Deering beneath the waves,they are treated to a demonstration of just some of the more remarkable features, built into the machine. From the cockpit, Dr. Deering can project the sound of his voice underwater. The machine's robot arms can bear the weight of sunken ships. It has a drill for boring through underwater mountains. It can also clear the sea floor via a powerful suctioning vacuum. Returning to the lab, Dr. Deering is about to showoff his latest breakthrough... a formula to allow man to breath underwater... when he is caught in a sudden explosion. Doused in the chemicals, Dr. Deering begins to choke, no longer able to breath the air. Aquaman rushes him to the ocean. Once submerged, Dr. Deering realizes that he has become a water breather. An examination by a local doctor bears no fruit. There is no cure for Dr. Deering's condition. The revelation begins to unravel Dr. Deering's mind. He despairs that he will never finish his work. Fears the isolation of living alone beneath the waves. Aquaman moves Dr. Deering into a water tank, on dry land, hoping to allay his fears of residing in the ocean's depths. Dr. Deering, though, merely feels like a freak. Dr. Deering is able to survive outside of water just long enough to reach his lab. Therein, he puts on a diving suit, first filling the helmet with fresh water. Through the night, he toils in the lab, making modifications to his ocean-going machine. With the cockpit sealed tight and filled with water, Dr. Deering drives his machine into the city. Once there, he begins to steal the skyscrapers.
The local police mount an offensive against Dr. Deering, but find they are powerless to stop his machine. It is impervious to gunfire. It's telescoping robot arms have enough reach to pluck the police helicopters from the sky. In the end, the officers can only stand and watch, as Dr. Deering drags skyscraper after skyscraper beneath the waves. Arranging the stolen structures on the sea floor, Dr. Deering builds an ersatz underwater metropolis. His city, though, lacks just one thing. A population to live in it. To that end, he returns to the surface, and attacks a passing ocean liner, effortlessly lifting it above the waves, with the machine's powerful robot arms. Aquaman summons two great whales, that he then rams into Dr. Deering's machine. Freed from the machine's grasp, the ocean liner slips back into the sea. Aquaman and Aqualad begin distracting tactics to occupy Dr. Deering, giving the ocean liner time to escape. Aquaman and Aqualad, however, do not. Dr. Deering is able to get a grip on the two heroes in each of his machines robotic arms. Steel cables drop from the arms, encircling Aquaman and Aqualad's extremities. They are left to dangle like puppets. The two now serve the deranged Dr. Deering, as the newest residents of his empty underwater "city".
Lost in his dementia, Dr. Deering greets Aquaman and Aqualad as neighbors, remarking on the nice weather they're experiencing. Suddenly, a gigantic pair of scissors glides out from behind a skyscraper, bearing down on the trio. Dr. Deering flees into his machine. The scissors, however, actually target the cables, snapping them loose from the robot arms, freeing Aquaman and Aqualad. The scissors turn out to be the hard water constructs of Mera. She has arrived to offer aid. Dr. Deering attempts to recapture the heroes, but Mera entangles his entire submersible within ribbons of hard water. Dr. Deering draws on deep reserves of power, to enable his machine to break free. Activating the underwater vacuum, Dr. Deering tractors the three heroes back to his vessel. Aquaman instructs Mera to create a hard water sphere to plug the suctioning mouth, allowing them to escape it's pull. Dr. Deering gives up the fight, deciding instead to resume his efforts to populate his underwater city with surface dwellers. He returns to the surface, bearing down on another passing ocean liner. Mera creates a water spout that raises the ship out of the reach of the submersible's robotic arms. At which point, Dr. Deering finds himself under attack by a US Navy gunboat. Aquaman pleads with the Navy captain to call off the attack, citing that Dr. Deering is not in his right mind. The US Navy deems Dr. Deering too dangerous to be left to his own devices. Aquaman is given one hour to find an alternative solution, before the Naval captain resumes the assault. Aquaman sets a plan into motion. He confronts Dr. Deering, telling him that he can stop pursuing surface dwellers, as his "city" is already overpopulated. Dr. Deering travels back to the "city" to find that it is, indeed, teeming with people. In actuality, they are from Mera's home, Dimension Aqua. Stepping outside of his submersible to investigate, Dr. Deering is jumped by Aquaman. Dr. Deering, however, has a portable, handheld controller for his machine. He captures Aquaman and Aqualad, again, in the great robot arms. Dr. Deering, though, suddenly begins to drown. The water breathing formula has worn off. With Aquaman and Aqualad held fast in the grip of the machine's robot arms, there's no way for them to get Dr. Deering back up to the surface. Aquaman summons a group of electric eels that short out the portable controller. The robot arms relax their grip. Aquaman saves Dr. Deering, whose sanity has returned with his ability to breath air. Aqualad uses the submersible to return all the hijacked skyscrapers to their origin points.