The Duck Family is traveling deep into the ocean to look for the City of Mervana by following clues left in Isabella Finch's journal. Everyone is excited for the adventure, except for Louie, due to them constantly being put in harm's way by the various societies that they meet on their adventures, and Della, who cannot stand being near anything fish related. They find Mervana and are greeted by the hippie-like society of merpeople. Della stays on the submarine as the rest of the family is led through the city by Aletheia and Vero who tell them that they were once ruled by King Honestus who brought his people to live under the water permanently to escape society on land. He mysteriously disappeared while trying to "find his own truth" and the Mervanans await his arrival.
While Scrooge, Huey, Dewey and Donald take part in designing their own mer-tails, Louie becomes suspicious of a cave that they are told to stay out of. Believing something sinister is afoot, he decides to investigate with Webby, the ever optimist, thinking he is being paranoid and joins him to prove him wrong. They are soon chased by a giant fish monster, which Louie believes that they were going to be fed to, but are rescued by Beakley who feigns agreeing with Webby's sentiment. She privately tells Louie that Webby should always believe that everyone has good in them, otherwise she would become downtrodden.
Huey, Dewey and an enlightened Donald are able to successfully design their own mer-tails, but Scrooge cannot come up with a design due to his impatience and cynicism. Della tries to wrestle with her fear of fish which is further complicated when Dewey arrives to show off his tail. Louie, Webby and Beakley finally enter a sunken part of a castle and find the singing Magic Harp who is only capable of telling the truth. She outright reveals that Louie is correct in his suspicion and tells them the whole story.
The Mervanans used to live on both land and sea and were led by the newly appointed King Honestus. The Magic Harp tried to warn Honestus that while being in the ocean is fun, they must also work on land. Honestus ignored her and refused to lead his people, so he escaped into the ocean while the kingdom on land crumbled. Due to Honestus staying in the ocean for a long time he transformed into the fish monster that they had encountered earlier. Furthermore, the Harp is unsure if the merpeople are actually going to feed them to Honestus.
Webby is shocked by the truth and is further upset to learn that Beakley is also doubtful of the Mervanans' plans with them. They take the Harp and race back to the society to warn them. The Mervanans deny wanting to feed them to a monster as Honestus arrives and begins attacking everyone. Aletheia and Vero are horrified that their ideal society is falling apart. As they share Webby's feelings, Louie gives them a pep talk and tells them that sometimes they need to look for the best in people.
The Mervanans fight off Honestus while the Duck Family and the Harp race back through the cave and up in the collapsing tower. With the help of the Mervanans, they race to shore with Honestus chasing them, only for the tower to fall and crush him. As they make it to shore, the Mervanans stand on land and regrow their legs, though they have trouble remembering how to stand. An injured Honestus pulls himself to land and Della arrives kicking him down one last time, while partially overcoming her disgust of fish.
Honestus returns to normal and still does not wish to be king, but Aletheia and Vero tell him that they have changed their society so that he does not have to be if he does not want to. Together, the Mervanans and the Harp rebuild their kingdom on land and Scrooge considers it an accomplishment that they told "shiftless hippies to work hard and get a job". Webby and Beakley make up with the latter telling her that following this adventure, she will keep no more secrets from her. However, as soon as they enter the submarine, the Harp states that Beakley is indeed lying to Webby.
- David Tennant as Scrooge McDuck
- Danny Pudi as Huey Duck
- Ben Schwartz as Dewey Duck
- Bobby Moynihan as Louie Duck
- Kate Micucci as Webby Vanderquack
- Tony Anselmo as Donald Duck
- Paget Brewster as Della Duck
- Toks Olagundoye as Mrs. Beakley
- Retta as the Magic Harp
- Hynden Walch as Aletheia
- Greg Cipes as Vero
- Jeff Bennett as King Honestus
- "The Truth"
- Webby's line "Look at this stuff, isn't it neat?" is a direct reference to the song "Part of Your World" from The Little Mermaid.
- The episode reunites Greg Cipes and Hynden Walch who also voice Beast Boy and Starfire, respectively, on Teen Titans and Teen Titans Go.
- All the main Mervanans have names that mean "truth". This is pointed out by both Aletheia and Vero whose names originate from Greek and Italian, respectively. Honestus is a pun on the word "honest". This reflects how the Magic Harp is a "living lie detector" that can detect when someone is lying.
- Della is revealed to suffer from ichthyophobia, the fear, or disgust, of fish.
- This is the first episode to have the full version of the Season 3 version of the DuckTales Theme. It includes Della in the scene of the treasure room before the dragon blows out the torch in Scrooge's hand.
- The Harp of Mervana reveals that Mrs. Beakley is lying to Webby. This is the first hint towards Webby's true identity, something that would be fully revealed in the later series finale "The Last Adventure!".