Mermista is using her powers to summon fish into the nets of the rebel fishermen, when she receives a distress signal from a whale. She goes to aid the whale, as Adora transforms into She-Ra and swims after her. Hordak has stunned and captured a baby horned whale, intending to present the whale to Horde Prime as a present, because Horde Prime collects strange creatures. She-Ra helps Mermista free the horned whale by stopping Hordak's escape, giving Mermista time necessary to free the whale.
Together, they take the whale to Mermista's undersea kingdom, Salinius. However, as they leave, Hordak is able to attach a limpet spy device to the whale. When Mermista arrives at Salinius, her father, King Mercia, scolds Mermista and reminds her that it is forbidden to bring a "dry-lander" to the Sunken City. However, when he learns that the visitor is She-Ra, he permits her to stay, having knowledge of her feats of heroism. The legend of She-Ra has indeed reached even the Sunken City. King Mercia explains his fear that "dry-landers" are so obsessed with greed that they might one day make an effort to steal the Power Pearl, which serves as the source of the Mer-Peoples' power. Without the Power Pearl, their legs cannot become fins, nor could they communicate with the creatures of the sea.
Hordak spies on this conversation, and is intent on stealing the Power Pearl and tapping its power. She-Ra and Mermista discover the spy device, just as Hordak cracks the dome surrounding the Sunken City and steals the Power Pearl, causing flooding. The Mer-People will not drown, because they can breathe underwater, but without the Power Pearl, they will not be able to swim to safety. She-Ra is able to avert the danger by reaching into the mind of a giant sucker-clam with her telepathy and asking it to allow her to move it. She plugs the hole in the dome using the sucker-clam. Hordak, meanwhile, is attempting to tap the power of the Power Pearl and use it to summon mighty sea creatures to serve his whim.
She-Ra, Mermista, and the baby horned-whale arrive at Hordak's ship just in time to face a giant sea Crabulus that Shadow Weaver has summoned using the Power Pearl. While She-Ra deals with the threat, Mermista and the baby horned-whale leap over the ship and recover the Power Pearl. Hordak unleashes a barrage of stun charges and freeze torpedoes, but having recovered the Power Pearl, Mermista's power to communicate with the sea creatures is restored, and she uses it to summon an angry creature: The baby horned-whale's mother, a massive horned-whale. The large creature devours Hordak's ship whole as Hordak and Shadow Weaver escape. King Mercia rewards She-Ra and the baby horned-whale with a medal of honor, and has himself learned a lesson about trust and prejudice.
Synopsis by Michael Cravens