Nature has given us shelter and life for humans, but people often destroy what they have, without acknowledging what they have done in the past. At the port of Mari is the kingdom of a statue of the goddess Shil, who ruled over the mermaid gods, and lately the soldiers from neighboring countries often invade the port area to seize the oil pipelines at sea. A group of mermaids living under the harbor island sent a message to the sea king, who lived for thousands of years in the underwater world, to solve this problem. The Sea King, along with her daughter Mila, led the secrecy of the waterway when she received permission from her father with the spear of lightning and the power of the storm. The port garrison was suddenly struck by lightning, and the sea waves destroyed enemy ships for a short time. Captain Sam, who was injured and fell into the sea, but just as he was about to pass out, saw a beautiful woman with a fishtail swim in to help him.
Princess Mila took Captain Sam to her castle under the sea which he woke up surprised to see sights he had never seen before. He looks at the princess and thanks her for helping his life. The princess struck up an open conversation and said that the outside world was full of creatures and new things that she wanted to learn, but her father never allowed her to go out only on missions. Captain Sam replies to her that not everything is like what she heard outside, it's full of risks and especially cunning people everywhere. He believed that the sea is vast and full of nature which, like another world, has the freedom to swim anywhere while living carefree. Suddenly, the ringing bell in the middle of the hall called Princess Mila to join the meeting because more urgency than the volcano under the sea will explode soon and the big wave will destroy human life. Captain Sam asked for permission to join the meeting because he has the idea to stop Volcano from exploding. Princess Mila though a little and chords but she used her magic to change his legs into a fishtail because her fish people never trust humans. In the human world, after the war, the army of Mari Kingdom rebuilt the port and tried to find Captain Sam who lost at sea.
The Sea King gave the power of the Heart of the Sea to the team of princesses and a new member, Captain Sam. She did not tell her father the truth that Captain Sam was human. On the way to the mountain in the seabed, a group of sharks attacked them due to the seabed waves causing the sharks to lose consciousness and hit the princess, causing injuries to her hand. By a group of male soldiers secretly Crushed on the princess, he used seaweed with medicine to wrap the wound on her arm. Captain Sam watched these actions, feeling upset, but he said nothing. The princess took out the sea heart music box to make the sharks and fish turn back to normal as the sea's melodic lullaby. They continued forward until they saw a volcano erupting, and the captain Sam told the Fish Army to dig many ways to keep lava from flowing into a deep pit that could turn water into gravity and made giant waves. Everyone worked hard, but the results were as expected. As the end of the first eruption, the seabed began to shake again, this time with bouncing rocks, causing their group to retreat.
Princess Mila was so tired she couldn't swim, so Captain Sam, who was swimming beside her, hugged her to protect her from danger, the bubble under the ground where they stood, pulling them close to the mouth of the volcano. They saw clearly that this time it was not lava, but a group of jellyfish trapped under the pit under the magma, a rare and legendary species of jellyfish came out of it. They watched the jellyfish enjoy their freedom and swam around them, shining beautifully. The sea king knew truth that Captain Sam was human by the soldier's report, but The sea king did not sanction him because Captain Sam saved his daughter's life and built many virtues. The fish people also changed their minds, started to think well of people and performed the same good luck rituals as in the past for festival-goers. For the unsupported and disgruntled soldier who reported has fled to another kingdom, Captain Sam has decided to be a fisherman beside Princess Mila and protect her with love, waiting for the day of their marriage. He and Princess Mila also swam under his old friend's ship to announce that he was alive and about to get married, making everyone happy, especially as everyone also agreed to hide the secrets of the world of the seabed. On the wedding day, a group of crabs and lobsters play music, with a dancing group of jellyfish. The Sea King gives the power of the Heart of the Sea to his daughter as a wedding gift. Captain Sam dances with Princess Mila in the middle of the giant pearl hall full of happiness and promised to love her forever.