The family joins a zen mermaid society to find a Missing Mystery, but Louie is convinced the seemingly innocent mermaids have a villainous agenda of their own.
Inside of a submarine, Huey begins to explain about the city of Mervana that was originally thought to have vanished beneath the sea. But with the help of Isabella Finch's lost journal, it would allow them to discover clues within Mervana that would lead them to one of her missing mysteries known as The Lost Harp of Mervana. While Scrooge reminds everyone that this is Huey's adventure, he nevertheless cuts Huey off on his presentation. The both of them continue to talk back and forth about how when the Mervanans disappeared into the sea, the harp disappeared along with it.
They then state that legend has it, Mervana's citizens were made up of half-man and half-sea creatures, whose legs could turn into fins underwater, allowing them to live on both the land and sea. While Louie grows concerns for their safety and Webby expresses her excitement, Della shows her disgust for fish on a high enough level in which she accidentally loses control of the submarine, forcing Donald to take over. Webby begins to question Louie on why he always assumes the worst, with Louie reminding her of all the times they've been used as sacrifices, but Mrs. Beakley soon joins in and compliments Webby's positive attitude.
Suddenly, Webby notices mermaids from one of the submarine's windows, as the family huddles together around the window mesmerized at the two mermaids floating in front of them. However, they are soon distracted when they notice Della arming the submarine's torpedo's out of immediate hatred for the mermaids. Afterwards, the family gear up into their scuba diving suits and make their introductions to the mer-people, who introduce themselves as Aletheia and Vero. They then state that Mervana is a place free of suffering, but Scrooge disagrees, believing suffering can build character.
When Dewey offers his mom, who's still inside the submarine, to join them on the trip, Della assures him that she'll be fine where she is and would rather stay behind than be revolted by anymore sea life. As the family pursue their aquatic adventure among the other mermaids, Aletheia and Vero explain to them how the citizens of Mervana believe in finding your own truth, in which you would then give up all your belongings in order to live peacefully beneath the sea. Although Scrooge expresses irritation towards their beliefs, they soon arrive in the main cave of Mervana.
They then share some history about King Honestus, and how although they grew to adapt to both land and sea life, Honestus led the citizens of Mervana underwater to escape the troubling conflicts of land living. When Louie questions Honestus' current whereabouts, Aletheia claims he one day left to find his own "truth" and never returned, as it's believed by his fellow mermaids he became one with the ocean. In his honor, Vero states the Mervanan's continue to seek out their own truth and those who are proven worthy can learn all of Mervana's secrets.
Once Aletheia and Vero leave in preparations to help the family with finding their own truth, Huey shows interest as he believes this way they can track down the Lost Harp of Mervana, but Scrooge continues to have an unfavorable impression on the city and its lifestyle. Soon Louie begins to look around, as he notices a strange looking cave while explaining to Webby his suspicions on how everything seems too perfect, no one claims to have heard of the harp, and the cave they're in may be a sacrificial platform for Honestus to be fed. Even so, Beakley arrives and tries to lower his doubts by believing the mermaids to be harmless, having had experience with their kind during her previous years as a spy.
Webby continues to attempt to convince Louie that everything's fine, but when Louie shows her the mysterious cave with danger signs sprayed all over the entrance, he remains convinced the mermaids are hiding something as he makes his way into the cave with Webby following him. Meanwhile, Donald, Scrooge, Huey, and Dewey begin to play soft drum beats on bongos, as Vero soon shows them where to make their ceremonial fins. Scrooge, however, continues to remain frustrated as he rushes to draw on his fin, leaving Aletheia unconvinced in his journey to find his truth.
Meanwhile, as Louie and Webby make their way deeper into the cave, they suddenly hear a mysterious voice calling to them. When they carry on to investigate the noise and mysterious pile of bones lying around, they are soon shocked to discover a large sea creature trying to capture them while leaping back and forth into the water. Eventually, the creature begins to chase them on land, soon leaving Louie and Webby at a dead end. Just as the sea creature's about to attack them, Beakley lowers a rope from above and hooks it onto Webby and Louie, hoisting them up to safety.
As the sea creature backs away, Webby asks Beakley why she's also in the cave, with Beakley claiming to have been merely meditating as she asks Webby to take the lead. With Webby out of hearing range, Beakley admits to Louie she also believes something strange is happening, but she doesn't want Webby to know out of fear of breaking her spirit if an entire society she knows and loves turn out to be liars. Louie continues to feel pessimistic about Beakley's beliefs, but nevertheless they try to remain calm once Webby returns to them. As they continue down on the path, they soon enter a large tower with vast amounts of steps to the top, much to Louie's distress.
Meanwhile, as everyone finishes up designing their fins, with Donald's standing out the most to Vero and Aletheia, Dewey decides to show his new fin to Della, only to soon fall back into the water on his way. However, Vero and Aletheia still believe Scrooge to be unworthy of the secrets to Mervana, due to his continuous negative energy and judgmental attitude towards everyone. Back on the submarine, Della tries to alleviate her boredom with a game of tic tac toe, when all of a sudden Dewey enters the sub and shows her his new fin, leaving her nauseous once more.
Meanwhile, as Beakley, Webby, and Louie finally make it to the top of the tower, they soon hear the mysterious voice again coming from an object covered in a blanket across from them in the dome-shaped room. They soon uncover the blanket and discover the lost harp of Mervana, which also reveals to be a mystical lady that will only ever speak the truth, and can tell when others are lying or speaking honestly. Webby then requests from the harp to assure Louie that nothing sinister is happening, but she refuses since she would be lying if doing so.
She then tells a tale in a singing format that long ago when King Honestus was crowned, the harp suggested he looks after his work on the land rather than constantly remain distracted by the sea, but he instead chose to ignore her advice out of laziness. Eventually the towers and monuments on the land began to crumble and break apart due to lack of any care for sustaining its structure. The harp continued to plea to Honestus by admitting the truth that his choice to do nothing was only making life on the land worse, but Honestus still refused to listen.
Eventually, the citizens of Mervana looked to Honestus for his leadership in helping improve their land, but Honestus instead chose to abandon his duties and flee back into the sea. This caused almost all the towers upon the surface to sink into the sea, and the longer Honestus remained under the sea, the more monstrous his body and mind began to transform. This leads Louie, Webby and Beakley to realize the sea creature they saw earlier was actually King Honestus all along, while coming to the conclusion that even the harp is unsure whether or not the Mervanan's intend to use them as sacrifices.
However, Webby remains in disbelief of what the harp is telling them, and although Beakley tries to lie by saying her story is misjudged, the harp claims her to be fibbing non-stop. Eventually Beakley grows agitated while shouting at the harp about her true feelings that you should never trust anyone including the mermaids, and that even she tends to lie to Webby. his leaves Webby saddened, but Beakley in her defense claims she was merely trying to protect her from the harsh truth that not everyone in the world has good intentions. Even so, Louie suggests they bring the harp back downstairs to show the other Mervanan's the truth.
Meanwhile, Scrooge continues his attempts at living truthfully to learn about Mervana's secrets, but when asked by Vero to give up all of his worldly possessions, Scrooge refuses, believing the idea of giving up all your hard earned work to be complete nonsense. But to Scrooge's surprise, Vero and Aletheia are impressed by his statement and accept him into Mervana, believing he's now found his own truth. After a small moment of celebration, Scrooge demands the location of the lost harp of Mervana, but Vero admits he and the other Mervanan's are unsure themselves, believing one day King Honestus will return to them and reveal all the secrets of Mervana.
Back at the submarine, Della strives to head into the water with the hopes of catching up to her family, until suddenly the sea creature from earlier begins roaming around the submarine, causing Della to rush back inside. Meanwhile as Beakley begins to carry the harp, she orders Webby and Louie to run ahead and warn the family about the truth of Honestus. However, Webby's newly depressed state causes not only her inability to move or walk, but also to slow Louie down as he attempts to drag her along. By the time they reach the bottom of the tower, they are once again approached by the sea creature and flee back towards the entrance of the cave.
Louie then shouts to everyone in the main cave that a sea creature is coming, but is too late as the monster then leaps into the air and begins attacking the mermaids. Scrooge demands the harp's location from the sea creature itself, only to then witness Beakley carrying it in front of the cave entrance. Desperate to slow the monster down, Dewey attempts to convince Donald he should reveal his anger side, but Donald is still stuck in a trance of finding his own peace and truth. However, once Dewey tells Donald that his famous chili is just okay, Donald immediately grows furious and manages to headbutt the sea creature back into the ocean.
Meanwhile as Webby, Vero, and Aletheia sulk over their feelings of betrayal, Louie tries to convince them that even though Honestus' actions were a lie, they and the other mermaids built a society based on truth that may not be consistently good, but it's not just a place and it will always remain in their hearts. Although Webby initially doesn't believe him, Louie reminds her that someone once told him how it's better to see the good in people rather than assume the worst. Meanwhile as the sea creature closes in on the harp and the rest of the family, the mermaids assist in the battle and manage to fend off the monster long enough for the family to flee back into the cave.
However, the harp gives away their location when asked by the sea creature, leading to another chase through the caves, but luckily they manage to get back to the tower before it arrives. However, the sea creature then begins hitting the walls of Mervana, weakening the structure further and causing numerous debris to fall into the sea. Once the family make their way to the top of the tower, they then dive into the sea in order to get to the beach for safety. While the tower continues to fall apart and the family swims towards the beach with the help of the mermaids, the sea creature soon crashes through the tower wall and catches up to them. Just as it's jaws open up preparing to strike, a large debris from the top of the tower falls straight onto the sea creature, dragging it deep beneath the sea.
Once the harp and the family make it ashore, the Mervanan's also manage to transform their fins into legs for the land, but unsurprisingly have trouble remaining stable at first as most of them collapse on the ground. Out of nowhere, the bruised sea creature returns once again, but Della manages to arrive just in time to kick the monster away from everyone. Suddenly with the sea creature now on land for long enough, he transforms back into his true self as Honestus. The harp then belittles Honestus for continuously trying to stay away from his problems, and the only way to fix the city of Mervana now is with true hard work.
Nevertheless, Honestus begins whining about his duties as a king being too difficult to succeed in such a task, but Aletheia and Vero assure him that all the Mervanan's will assist in the duties to help rebuild the city. Honestus agrees and the mermaids begin repairing the newly broken tower, while the family decides to leave the harp behind to help show the Mervanan's the hard truths so they can succeed in future endeavors. Webby and Louie then remind King Honestus that although their world isn't perfect, it shouldn't be considered all bad either and instead be seen as a good balance between them.
Believing their work to be done in Mervana, the family make their way back into the submarine to go home, as Scrooge remarks the harp to be back where it belongs for the sake of the mermaids and their attitude towards hard work. Webby then adds that the harp wouldn't be of any use to them anyways since they're always honest with each other, as Beakley then assures her that from here on out she has no more secrets to be kept from her. Unbeknownst to everyone as the submarine lid closes, the harp can be heard from a far distance calling out Beakley's lie.
It is revealed that Della is scared of and disgusted by fish as she suffers from ichthyophobia.
This is further exploited when she says “I have no family,” when Dewey shows her his ceremonial fin.
Greg Cipes and Hynden Walch have worked together previously in Teen Titans and Teen Titans Go voicing Beast Boy and Starfire respectively.
This is even referenced by the coloration of both of their feathers and in the case of Aletheia; this is also shown by making her eyes green like the ones of Starfire. Vero also mention being vegan just like Beast Boy.
This is the first Season 3 episode to feature the full updated version of the DuckTales Theme.
Goof: When Beakley is playing the drums, she has her gloves on, but in the next scene, her gloves are gone.