This is the tale of a young mermaid who is being raised by her aunt. This young mermaid has the power to control pearls and make beautiful magic with them. One day her Aunt is called upon by a member of the royal family to help out at the official passing of the crown from the king and queen to the young prince since the death of the young princess. When the young mermaids aunt forgets her invitation to the royal party, the young mermaid takes off to give her the important paper. Along the way our new mermaid friend meets new friends and unknowing to her is part of a long lost secret and scandal that happened when she was just a baby.

Full Plot: The film begins with Lumina using her pearl magic to prepare for a fake coronation held for Kuda. Lumina is the bishop and Kuda is the princess that becomes 'The Queen of the Seven Seas', then they dance together.

Suddenly, Scylla comes and Lumina disbands the 'fake' party and hides the pearls she used. However, one pearl was left behind. In a hurry, Lumina tries to put it inside the shell, but Scylla sees it. She's trying to warn Lumina not to used her power in open sea and Lumina feels bad for it. In their cave, Lumina and Kuda continue their 'fake' coronation by customizing their hairstyles to mimic the princesses. They wonder what would it be like if they are the princesses and dream to see the castle. Scylla hears their chat, and hide a royal crest which should belong to the royal family, in a drawer.

In the castle sadly, King Nereus and Queen Lorelei have been having a hard time, still can't accept the fact that their daughter has been gone. General Caligo eases their pain and proposes them to befall his son as an heir to the throne, as Caligo is their cousin. The King and Queen see this as a rightful way and prepare a royal ball for that.

Although Caligo is very eager to realize his dream of making his son a king and being a hero himself, his son doesn't seem to have the same dream as he is. Instead, his son, Fergis, loves plants very much and acts awkwardly in his father's eyes.

Back at the cave, when Scylla is making a potion of some sort, an eel named Murray comes to the cave. He reveals Scylla's dark secret, the past between her and Caligo to poisoned the princess to set the main stage years later, which is now. At the same time, it's crystal clear, Caligo and his sidekick are planning a dire plan to kill the king and the queen. Scylla refuses, and warns Murray to go. Murray takes his offense, and threatens to tell everyone that she had killed The Princess. Having no choice, she has to answer yes to that.

Murray gives Scylla an invitation to the royal ball, and tell her to be at the castle on time. Suddenly, Lumina comes in and excited to hear more about the castle. This raises a suspicion about Lumina, which he thought Scylla was lived alone and Lumina's term of 'aunt'. Scylla throws Murray outside to hush him.

Lumina's excitement about the chat isn't over. She asks Scylla to take her with her to the castle. Scylla scares her off, telling all kinds of dangerous things she may encounters even if she's with her. Lumina has no other option, except to stay.

Disappointed, Kuda and Lumina play 'tail-tag' game. Kuda accidently knocks one of the bottle during the game. When Lumina is trying to pick it up, she sees the invitation to the royal ball. She is trying to give it to her aunt, but Scylla has gone far away. Lumina sees this as an opportunity to see the castle, and although Kuda reminds her of the threat her aunt gave to her and hesitated to go, Lumina pushed her to come with her.

Lumina noticed everytime Scylla comes back from the town, she always comes from a shortcut, then which appears as a dead end. Her intuition takes lead, and not long after, she found a passage through a skeleton of a big animal. While on the passage, Kuda accidently breaks the structure and collapse all in. They are lucky enough, but there's no time for relieve; All of the sudden, the second collapse happens. Again, they are lucky, but the invitation is lost to a narwhal.

Back at the castle, Caligo and Fergis are argueing. Caligo, in one hand, forces Fergis to do what he wants. On the other hand, Fergis doesn't want to be a king; Instead, he wants to be a botanist.

Not long after, Murray appears and informs Caligo that Scylla agreed. But, Caligo has one other concern he has to overcome, beside his evil plan; he wants his son to have a mate. This recalls Murray memories of Scylla's niece and proposes Caligo a pair which he may wants. This also raises a suspicion to Caligo. As long he remembered, Scylla doesn't have a niece. Then, Murray describes Lumina as 'blond and about 17' and even mention 'interesting coincidence'. Caligo is interested in and tell Murray to bring her to him.

Lumina and Kuda, in the other side of the ocean, is lost in the middle of kelp forest. Then they see the narwhal which took their invitation, only to lost it again. In desperate, Lumina sees the light in the distance and rush to it, in hope it is the city. But instead, they run into a vampie squid lair. Kuda and Lumina thought the baby squid is cute, but not after the squid attacks kuda. That's when they disturb a stonefish called Spike. Despite his gruesome look, Lumina helps Spike to cover his spikes with pearls and gives him advice not to scare people off. Spike comes with Lumina and Kuda to the city nearby.

At the cave, Murray alongside with two other eels, Wormwood and Garth, are trying to bring Lumina to Caligo but find no one. Regardless, Murray found an evidence that Lumina is the lost princess by a royal crest in a drawer which Scylla had hidden it from Lumina.

Back in the kelp forest, Lumina, Kuda and Spike are having a joke when Kuda is caught by a kelp. Spike cuts the kelp with his spike, and set her free. Then they come into the busy town nearby. Spike quickly gets along with the locals. Kuda notices Scylla in the distance, they're trying to hide from Scylla but instead come into Salon La Mer.

At first, Lumina is wrongly thought as a customer by Madame Ruckus, but when she notices Scylla is coming passing by, she disguises herself by changing her hair and so as Kuda. Madame Ruckus sees her talent and employs her alongside with two other employee, Sandrine and Cora. While Lumina is busy, back at General Caligo, his plan is arranged and is depended upon Scylla to make the poison. His plan is simple, let Scylla poisons the Merberry Nectar in the king's goblet, then he'll get the upper hand.

At Salon La Mer, Lumina has just become the most famous hairstyler in the city. Sandrine tries to keep up, but finally befriends with Lumina despites her cruel acts before.

Murray returns from his back-and-forth trip and tell Caligo he found no one. But also shows the royal crest to Caligo and confirms she is the princess. Caligo goes furious, as this could ruin his plan.

Again at Salon La Mer, Prince Delphin who has just come into the city is wrongly thought as an employee. He lifts things up and deliver it to Lumina. He quickly bumps into her. Later, General Caligo takes Fergis to Salon La Mer for hairstyling. Although he's been searching the lost princess, unknowingly, he asks Lumina to do the job. Fergis instead, is falling in love to Cora.

Later, Madame Ruckus comes to inform the trio that she got the invitation to the royal ball. Interestingly, they got anxious on what to wear. But Madame Ruckus winks at them. Then they go to the shop to buy some proper clothes. The shop is so crowded, in some ways that people just ramsacks every single clothes. So they decided to make them.

The movie skips to the royal ball, where Fergis calls upon Cora. His father interferes, then takes off his Quidest Fetere flower because it smells bad, then flaps his back. Cora and Fergis grow more closer after Cora states she loves his flower.

When Lumina notices the narwhal who took her invitation, General Caligo gets flapped by Lumina's tail, but he welcomes her instead, then telling he likes how she reorganized his son's hair. But, moments later, Murray tells him that Lumina is the lost princess. Meanwhile, Lumina gets a dance with Prince Delphin and chat together. Then Prince Delphin says he needs to go for a while, and makes Lumina promises to save the next dance for him.

Caligo takes no wasted time, he immediately drives Lumina outside the ball, and capture her and Kuda with the helps from the eels. Then they throw her to the dungeon. Lumina recognizes the eel as he aunt's friend. Murray tells her the truth; her aunt will poison the king. Lumina's creative minds let her steals the key from the eels and tied them together, Then she frees herself and Kuda.

At the ball, Scylla has swapped the goblet between Caligo's and The King's. Caligo knows this, but he got a dillema. If the king drinks the real nectar, and he drinks the poisons, it could be fatal. His dillema increases, when he has to decide which one is the poison: his or king's.

Then, The king coronates Fergis as an heir to the throne, and is about to have a toast. But, Caligo decides quickly to spill the king's goblet and offers him with his. Scylla tries to spill it, but blocked by a Trident. Lumina arrives in time to spill the goblet before the king drinks it, causing the hiss in the crowds.

As his plan come to an end, Caligo refuses to give up. He states Lumina is trying to attack the king and tells Tridents to arrest her. Scylla testimates, telling Lumina has just saved the king and the fact she was forced to do that by Caligo. But before she has a chance to say that, Caligo makes an effort to kill the witness and push her to one opened end of Spike. The venom of the stonefish puts an end to Scylla's life. But before she died, she tells Lumina, she took her from her parents to protect her.

Spike tells the antidote for the venom and turns out to be the Fergis's Quidest Fetere. But before he can say that, Caligo hushes him. Fergis decides the right, he defies his father and give Lumina the flower.

Caligo makes one last tries in desperation, he takes Scylla's body and intends to go outside the ballroom. Lumina gets furious; she chases Caligo before he leave the ballroom. But one Tridents hold her tail. Every Lumina's friends is now involved. They help defeated the Tridents Squad.

Before Caligo could reach the gate, Lumina sends her pearls to the gate, locked it and uses all the pearls in the ballroom to ensare him. Lumina gives the antidote and heals Scylla.

Scylla states Caligo is the one who ordered her to poison the king. And later on states Lumina is the lost daughter of the king and queen.

She revealed the story: Years ago, Caligo sought to be the king of Seagundia, and paid Scylla to poison the newly born baby of the queen, a true heir to Seagundia. She couldn't did it, so instead, she kept the baby with her, and told Caligo she was dead. As the child grew, she could have gave it back to her rightful parents, but she can't, and she feels guilty for that.

Fergis returns the Pearl of The Sea back to the king and gives it to Lumina. Scylla gains her forgiveness and stay together in Seagundia. The ball continues, and everyone (except the antagonists) live happilly ever after.