When Mera’s twin sister Hila is exiled with her lover Kandor for a crime that they did not commit, Kandor uses her to lure Aquaman into a trap to try to wrest from him the secret of the Seven Golden Eels. The Eels, when grasped, confer incredible powers on their holder. Aquaman refuses and runs a gauntlet of traps. Meanwhile, Mera’s water-shaping powers are restored by Professor Kreon of her extra-dimensional world. Eventually Kandor, Hila, Aquaman and Mera find themselves allied in battle against the menacing Armadillo Men. After their victory, they are approached by Aqualad, who tells them that Kandor and Hila have been cleared in their world. The two lovers decide to return home.
A playful Queen Mera, entices her husband, Aquaman, into pursuing her for a kiss. Catching up to her, Aquaman takes his prize, only the woman he kisses isn't Mera. It is her twin sister, Hila. As Hila is regarded as the black sheep of the family, Mera, out of shame, has never told Aquaman that she even had a sister, let alone a twin. Hila, along with her love, a man named Kandor, were exiled from Dimension Aqua, for crimes, she contends, they did not commit. Hila has come to ask her sister to free her from the hold Kandor has over her, so that she can start a new life. Hila leads Aquaman and Mera to the mouth of an underground river, one that leads to Kandor's lair, deeply recessed inside an underwater mountain. Posing as Hila, Mera takes the jeweled starfish, that Hila was sent out to obtain, and approaches Kandor's throne room. Aquaman hangs back, in the shadows, preparing to strike. Mera is startled to see a large amphibious creature sitting on the throne, and wonders why Hila had made no mention of Kandor's horrific transformation. The robed toad, however, is revealed to be a decoy, as the real Kandor appears, behind Mera, grabbing her. Mera calls out to Aquaman, but it is too late. Kandor has already trapped the Sea King.
Kandor and Hila had planned all along to ensnare Aquaman. Mera is furious that she fell for her sister's lies. Kandor, too, is angry, as Hila was supposed to return with Aquaman alone, not send Mera in her place. Kandor seeks the whereabouts of the "Seven Golden Eels", fabled to bestow great power upon whosoever grasps them. Aquaman refuses to give up the whereabouts of the eels. Kandor unleashes three deadly challenges to try to loosen Aquaman's tongue. First, one thousand and one harpoons are hurled at Aquaman. Aquaman dodges several volleys, before grabbing a harpoon and using it to block the remaining spears. Next, a pulverizing pendulum descends on Aquaman. Aquaman evades the first swing, but takes a glancing blow from the second. Thrown to the sea floor, Aquaman seems too stunned to avoid the deadly pendulum. At the last moment, Aquaman rolls away. The pendulum strikes the ground and shatters. Finally, Kandor sends in his armadillo warriors. Using Roman legionary tactics, the shelled soldiers deflect Aquaman's first attack, but are bowled over by his second. Curling up into armored balls, the armadillo warriors roll forward, knives out. Aquaman nimbly skips over their shells, sending the warriors crashing into the chamber walls. Angrily, Kandor, himself, rushes to engage Aquaman. Meanwhile, in Atlantis, Hila, impersonating Mera, sits upon the throne. Hila is taken aback by the outpouring of love and admiration the Atlanteans, and Aqualad, have for Mera. Overwhelmed, Hila bolts from the throne, and departs Atlantis with great haste. Inside the mountain, Kandor, using his native ability to control and manipulate water, has gained the advantage over Aquaman.
Caught on a column of hardened water, Aquaman is about to be crushed against the cavern ceiling, when Hila intervenes. Hila uses her own water manipulating powers to fight Kandor to a standstill. Mera, having lost her abilities some time ago, can only stand and watch, as Hila takes Mera's place at Aquaman's side, in opposing Kandor. In despair, Mera wanders Kandor's palace. Paused at a mirror, Mera is startled to see the image of Professor Xeron staring back at her. Through the looking glass, the chief scientist, of Dimension Aqua, telepathically restores Mera's powers. Kandor suddenly appears and gasses Mera, thinking it is Hila. The gas has hypnotic properties and places Mera under Kandor's sway. Seeing Hila, who Kandor believes is Mera, about to free Aquaman, from the clutches of Kandor's hard water starfish, he sends Mera to stop her sister. The twin sisters battle for several moments, constructing and reconstructing one hard water weapon after another. Mera wins. Kandor, realizing that "Mera" is actrually Hila, rushes to the side of his injured love. Aquaman asks Mera to quickly free him, but the gas has made her loyal to Kandor, so she refuses. The armadillo warriors return, pushing an enormous spiked wheel. They have chosen this moment to rebel against their enslaver, Kandor. Kandor hurls a massive hard water sphere at the juggernaut, which easily crushes the momentary impediment. Aquaman instructs Mera to create a hard water inclined plane. The juggernaut loses it's momentum on the ramp, rolling backwards to a stop. The armadillo warriors leap over the stalled juggernaut, and attack. Aquaman, Mera, and Kandor work together to defeat them. Hila recovers. Kandor frees Mera from his hypnotic control. Aqualad arrives to inform everyone that Hila and Kandor have been cleared of all of the charges against them. Their exile is over. They are free to return to Dimension Aqua.