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The story begins with unscrupulous fisherman John Mallick and his assistant Carlo using fishing bombs in open water to harvest coral reefs. As Carlo and Mallick gather their spoils, they suddenly notice something with a large tail thrashing about in the water. Mallick shoots the unknown creature, and attempts to pull the body onboard his boat, A Salt Weapon, but the vessel is hit by a powerful force. Mallick looks into the water and sees a figure surface, pointing a trident at him. He and Carlo escape without the body of the strange creature they had killed, which turns out to be a merman.
The person with the trident was the merman's eldest daughter, Diana, a beautiful but fierce mermaid with superhuman strength, and a magical trident that turns into a comb she wears in her hair. Although she loathes human beings, Diana ventures on to land for the first time to seek out her two sisters, Venus and June, to help her find Mallick and seek justice for their father's murder.
Venus, the middle sister, who divides her time equally between land and sea, is working in a cafe where she swims in a tank with a fake mermaid tail, and charms the customers with her siren powers. When Diana finds her, she is appalled by her sister's behavior, and suspects that there is another reason behind her choosing to perform for humans. June, the youngest sister, who spends much of her time on land, and has lived among humans for three years, is working at a sea park where she can communicate with the marine life and remain close to Randy, the man she is in love with, but who she cannot find the courage to confess to, as he has a long-term girlfriend named Cynthia.
Although the three sisters do not get along because of their differing life choices, they agree to band together to find Mallick and avenge their father's death. Over the course of the film, it is revealed that Venus has been forced to work at the cafe because its owner, Earl, has taken Venus' Birth Right, a pearl tiara - A magical item that is given to every merperson at birth, and leaves them powerless if it falls into the possession of another. Diana threatens Earl until he agrees to give up Venus' tiara, but she also finds a large seashell, apparently the Birth Right of another mermaid. Venus realizes that the shell belongs to Earl's middle-aged wife, Betty, who has forgotten her mermaid heritage after spending so much time on land. Venus gives the shell back to Betty, causing her to transform back into her younger mermaid form, and urges her to return to the sea. Betty, however, declines, because she now has a human daughter, Tessa, and instead asks Venus to keep her Birth Right safe. June theorizes that Betty now shares her daughter's human soul - an occurrence that only happens when there is love between a human and a merperson - and that she will now have an afterlife, something that other merfolk do not have.
Meanwhile, June and Randy are attempting to hunt down and trap Mallick together. At one point, Randy boards Mallick's boat, looking for evidence, but Mallick and Carlo knock him unconscious, and throw him into the sea. June, however, looks into the magic mirror that is her Birth Right, and sees that Randy is about to drown. She transforms into a mermaid, and drags him to shore, leaving him wondering who it was that saved him. It is gradually revealed that Randy is actually unhappy with his girlfriend Cynthia, as the pair have little in common, and Randy soon falls in love with June instead. June then accidentally reveals her mermaid form to Randy, but he is happy.
The three sisters then see on the front page of a newspaper that Mallick has discovered the body of their father on the seabed, and is keeping it in a freezer. They confront him, but Mallick has researched mermaids, and knows that they are bound to grant one wish to anyone who asks for it. He tells them to find him a chest of sunken treasure, to which they agree. After they bring him the gold, however, Mallick reveals that he wants to use the three sisters as the main attractions in a theme park. But his plans are ruined when the police arrive at his warehouse, following Randy's tip-off. When the police check the freezer, the merman's tail has transformed back into legs as the freezer is dry. They arrest Mallick on suspicion of murder, despite his protests that the three women are mermaids and the body is a merman.
At the end of the film, while Venus and Diana argue over whether they should keep the treasure, June is puzzled by the fact that Randy has started ignoring her again. Venus confesses to June that she used her hypnosis ability to make Randy forget all events of the previous few days in order to keep the sisters' mermaid heritage a secret. June is crushed, especially when she learns that Randy has proposed to Cynthia.
Despite this setback, June tells her sisters that she wants to continue staying on land, thinking that there may still be a chance for her and Randy. Venus wants to stay with her, as all her previous time on land has been as a prisoner, and she wants to explore it on her own terms. To the surprise of her sisters, Diana says that she will stay on land as well, so that she can watch over her younger sisters as their father would have wanted. The film ends on the implication that the three sisters will have many other adventures together.