The family heads to Mervana, an underwater society of mer-people, to find its lost harp. Two of the Mervanans, Alethiea and Vero, teach Scrooge, Huey, Dewey, and Donald their ways while Louie, Webby, and Beakley follow a mysterious sound. They eventually find it belongs to the harp, a sentient artifact that tells or confirms the truth, and learn that Mervana fell into the sea after its ruler, King Honestus, fled from his duties; turning into a monster. The harp also makes Beakley reveal the lies she told to Webby over the years, causing her to become depressed. Honestus attacks the family, but Louie is able to restore Webby’s confidence and help everyone fight him off until they reach land, where the Mervanans grow legs and Honestus turns back to normal. Afterward, the Mervanans tell Honestus that he does not have to shoulder his duties alone and they rebuild Mervana together while the family departs.