At the base of a lighthouse, Aqualad awaits the arrival of Aquaman and Mera. The old lighthouse keeper directs Aqualad's attention out to sea, where an enormous wave has formed into a giant saw. The wave crashes into the lighthouse, cutting it in half. As it topples, it is caught in an airborne cushion of water, that then moves out to sea, carrying the lighthouse. Aqualad dives into the ocean to follow it. He finds that the water stream carrying the lighthouse is being controlled by Mera. As it nears her position, she and Aquaman spy the lighthouse keeper hanging on for dear life. Mera flings him into the sea. Aqualad sends the octopus, Topo, to rescue the old man, and return him to shore. Confronting his friends on their menacing actions, Aqualad is quickly scooped up in a hard water "spoon", created by Mera. Aqualad dives out of the shallow ladle, only to find a much deeper hard water cup descending on him. At the last moment, the "cup" is shattered by another hard water object, and Aqualad dives deep beneath the waves to safety. He is met by the real Aquaman and Mera. Mera projects an immense hard water fortress atop the waves, from which Aqualad mounts an attack on the doppelgangers. Engaging Aqualad, the Mera duplicate's hard water constructs begin to shatter the walls of Aqualad's "fortress". The copy King and Queen of Atlantis, however, are surprised by Aquaman and Mera, who have flanked them and struck from the rear. Aquaman and Mera grapple with their duplicates. Suddenly, the duplicates fade away. In their place, standing on a floating platform, is a sinister alien, pointing his weapon at the monarchs of Atlantis.
The alien identifies himself as Xen, an intergalactic aquarium owner. He has come to collect Aquaman, Mera, and Aqualad as specimens for his exhibits. Focusing his duplicating ray gun on them, Xen creates giant energy duplicates of the three heroes, and sends them to collect his prize. The heroes dive deep, and once below the waves, Aquaman summons a squadron of octopi and swordfish to engage their pursuers. Aquaman's energy duplicate, however, countermands his orders, forcing the marine life to dance and perform acrobatic tricks. Aquaman and Mera suddenly turn, and charge their duplicates, striking them, then swimming , ever faster, in circles around the giant's heads. The constructs become too dizzy to continue the pursuit, but the three heroes have forgotten about the giant Aqualad duplicate. It grabs the lot of them and turns them over to Xen. At gunpoint, Xen leads the three heroes to his space ship, and an awaiting holding tank. Already within the tank are specimens from Jupiter, Saturn, Pluto, and Mercury. As the giant energy construct of Aqualad vanishes from view, the rocket's engines rumble to life. All seems lost, as Xen prepares to leave Earth.
As Xen's rocket begins to lift off, Aquaman summons a flock of sea eagles, commanding them to lift several octopi, out of the ocean, and over the ship. The octopi release their ink over the rocket's view shields. Xen reverses engines to return to the surface, rather than risk colliding with something before they can reach escape velocity. As the rocket descends, it lands upon sharp coral spikes that the octopi have pushed into position. The spikes crack the holding tank, freeing Aquaman and the other would-be aquarium specimens. All dive into the sea, with the Shell-Back Man from Saturn blocking the alien's assault weapons with it's massive shield-like shell. Xen turns the duplicating ray on himself, creating an army of Xen's to assist in retrieving "his" specimens. An epic battle ensues beneath the waves. While Aqualad, and the aliens, fight a pitched battle with the Xen army, Aquaman and Mera return to the surface, and Xen's rocket. Mera snatches Xen's duplicating ray gun away with a hard water hook. Aquaman uses the weapon to create a giant energy construct of Xen, and sends it below the waves to rout Xen's forces. Xen and his cronies are placed in the ship's hold. Aquaman creates a series of obedient Xen duplicates to pilot the rocket, and return the other aliens back to their respective worlds.