Two alien invaders in a flying saucer wish to conquer Earth for the expansion of their people, but since water is deadly to their race, they assemble a group of three super-villains—the Fisherman, fire-haired Karla and the invisible Un-Thing, known collectively as the Terrible Trio—to help them steal the seas from Earth with a satellite’s power. The Terrible Trio run afoul of Aquaman, Aqualad and Mera, but later ally themselves with the heroes against the aliens, who are destroyed by their own satellite. Aquaman turns the villains over to the authorities, but asks that they be lenient.
With the birth of his son, Aquaman contemplates giving up his life of adventure, much to Aqualad's chagrin. Just outside Atlantis, the Fisherman, the Un-Thing, and the Fire-Haired Karla make preparations to launch a satellite. Once activated, the satellite churns up the ocean waters, opening a mile high hole that extends to the sea floor. The Atlanteans rush to inform Aquaman of the situation. Investigating, Aquaman and Aqualad catch sight of the satellite, and give chase. The satellite is on the verge of escaping them when it is caught in one of Mera's hard water constructs. Aquaman deactivates the satellite. Bearing witness to Aquaman's interference, the Terrible Trio take action. The Fisherman casts his line around the satellite and reels it in. As Aquaman and Aqualad move in to engage the Fisherman, they are blocked by the Fire-Haired Karla, who sets the sea ablaze around the two heroes. Mera is just about to enter the fray, when she finds herself held fast in the grip of the invisible Un-Thing.
The invisible Un-Thing drags Mera, and the baby, into the Terrible Trio's underwater saucer. Aquaman summons a thresher shark to serve as a barrier against the Fire-Haired Karla's acetylene torch-like hair, and leads Aqualad to safety through the flames surrounding them. Aquaman descends on the underwater saucer to rescue Mera, and his son. He is attacked by the invisible Un-Thing. Unable to see where the Un-Thing's blows are coming from, Aquaman is beaten down. The Terrible Trio escape with Mera, the baby, and the satellite. Re-activated, the satellite begins carving holes in the ocean again. Aquaman and Aqualad catch up to the underwater saucer, and re-engage the Terrible Trio. Aquaman commands octopi to squirt ink into the water, allowing him to target the invisible Un-Thing. The Fisherman catches Aqualad in his line, and begins to reel him in. As Aquaman struggles with the Un-Thing, they both plummet into one of the open holes in the ocean. No longer immersed in water, the Un-Thing loses his invisibility. His advantage gone, Aquaman easily defeats him. Meanwhile, Aqualad, instead of fighting against the Fisherman's efforts to reel him in, begins swimming, ever faster, around and around the Fisherman, entangling him in his own line. Disgusted by the incompetency of her two associates, the Fire-Haired Karla resumes the attack. Her assault on Aquaman is blocked, however, by one of Mera's hard water constructs. The Queen of Atlantis is free.
The Fire-Haired Karla proves able to burn through Mera's hard water barrier. Mera, changing tactics, grabs the Fire-Haired Karla in a hard water hand. With the Terrible Trio defeated, their masters make their presence known, as an alien saucer descends down one of the ocean holes. Known only as "Them", the aliens intend to take the earth for their own people. Water, however, is anathema to them, so they enlisted the aid of the Terrible Trio in stealing every drop of water on Earth. To maintain the clandestine nature of their invasion, the aliens have decided to eliminate Aquaman, Aqualad, and Mera, themselves, along with the Terrible Trio. Aquaman ushers everyone into the Terrible Trio's underwater saucer, avoiding the alien's first assault by escaping into the sea. The aliens pursue the second saucer into the ocean. Using the satellite controls on board their saucer, the Fire-Haired Karla sends the satellite on a collision course with the alien saucer. Both are destroyed on impact. Aquaman turns the Terrible Trio over to the authorities. In appreciation for the Fire-Haired Karla's assistance in defeating the aliens, Aquaman offers to speak on her behalf at trial, in the hopes of garnering her leniency from the judge, during sentencing.