Hana, the last generation of mermaid Princess , sneaks out to visit the danger zone that her father forbade. She was also stopped by a group of lobsters and sharks. But Hana disguised herself and the seaweed to enter the restricted area. She wants to come here because she hears her people talking about people is building underwater railways, and especially the waterway system that can flow into the sanctuary with the moonlight building. And that building is keeping the mermaid queen ring that was there as well. Hana wanted to get the ring back to her mother because her mother had lost it while rescuing a group of people from the bottom of the sea. And it is the only ring that represents the love between her father and her mother. On the other hand, Hana wanted to give the ring as a birthday present to her mother as well.
Hana shrinks like a baby fish, swims into an underwater sewer until the bend turns, and the wind blows the water pumping her into the other side of the waterway. Because the force of the pump is too strong, it causes her was fainting and drowning. Faz rode a bicycle, took newsletters distributed through homes in the city. He also came to the water supply building, which converts seawater into pure water, and gave his father a school bill letter. And his father owned the water company. Because Faz wanted to living a simple life on his own, he looked for a part-time job while on vacation from school. Even though his father forbade him not to do small things, he did not listen because he wanted to train himself to learn the basics and become a strong person.
Hana, who is in the gutter under a worker who is walking, tries to use her special instincts to ask for help. But no one heard her voice until Fez passed through the area and heard a strange sound coming from a manhole under his feet. He bent down to see a beautiful blue-and-purple baby fish stuck in it. So he hurried to help the baby fish by taking a glass of water from the worker who was hanging by the side of the road, put water into the glass and dropped the baby fish in it. He thought that maybe he was too tired to hear the baby fish speak human language and just stared at him with wide eyes. After handing over the bill to his father, he hurried home to adopt the baby fish.
As he put the glass of fish on his table, he heard the baby fish talking again. And this time he saw the baby fish turn into a mermaid, and she asked him to put her in a shower tube . So he took her to his bathroom and Hana turned to a big mermaid . They are conversation each other and Faz agrees to help her get the ring. By their plan and they will be held in the Water Festival, which the officials will celebrate in the area of Moonlight building. For days now, he has been taking Hana on a city tour by putting her in a water bottle. And Hana also fell in love with Faz because he gave her a warm and caring feeling. When she saw a group of girls talking to Faz, she sprayed water on them in jealousy. Faz also fell in love with Hana when she always encouraged his work and his dreams.
The day the festival began, Faz met his father, talking to environmental officials. Therefore, he can sneak into the secret room while everyone is busy watching the show. Faz managed to get the ring and hurried out, but the glass he had placed near the entrance had disappeared. And when he asked the children nearby, they saw a grandmother take the bottle to the left side . So he hurried to find out that the grandmother had taken his glass of fish with Hana in it and poured it into her aquarium. And those fish are chasing Hana. Faz quickly reached out to grab Hana, who was injured and his hand was also bitten by the big fish. Hana opened her eyes and saw that on her fish tail was a bandage and there was a ring next to her. She tried to push herself out of the water to kiss Faz, who was asleep on the table. But Faz woke up and they were both shy. The next morning, Faz's father found out that his son had stolen the ring and came to his son's place.
Faz's father is shocked to find out the truth and agrees to help Hana get back on his company waterway system. Hana says goodbye to Faz and promises that one day she will be back. After her mother's birthday party was over, her mother noticed that Hana, who was silent, came to ask her daughter what had happened. hana confesses telling everything to her mother. Therefore, her mother used her special energy to her to become a human and study at the Human World School, but had to come home once a year. Hana meets Faz as promised and they become the happiest couple.