Unscrupulous fisherman Mallick, Sean Taylor and his assistant drift into perilous waters as they hunt coral reefs with soda bombs. When a large creature thrashes in the open sea, Mallick fires and discovers a merman among the depths. They struggle to pull the body aboard, but a powerful force rocks their vessel, scattering any trace of the creature. A figure rises from the water, raising a trident toward Mallick, and he and his unnamed helper flee, leaving the body behind in the restless waves.
The figure is Diana, Erika Heynatz, the merman’s eldest daughter. She possesses superhuman strength and a magical trident that can morph into a comb she wears in her hair. Diana buries her father’s body and, though she loathes humans, she ventures onto land for the first time to enlist her sisters Venus and June in pursuing Mallick and seeking justice for their father’s murder.
Venus, Nikita Ager, works in a cafe and swims daily in a tank with a fake mermaid tail. When Diana finds her, Venus is taken aback by her sister’s demeanor and suspects there may be more behind her decision to perform for humans than simple necessity. The youngest sister, June, Serah D'Laine, has lived among people for three years and works at a sea park. June is involved with Randy, Daniel Frederiksen, a marine patrolman who has a long-term girlfriend named Cynthia, Lara Cox. Although the three sisters clash over their different life choices, they agree to unite to track Mallick down.
June and Randy team up to pursue Mallick, aware of his illegal fishing. At one point, Randy boards Mallick’s boat to gather evidence, but Mallick and his partner knock him unconscious into the sea. June rescues him and drags him ashore, and it gradually becomes clear that Randy is unhappy with Cynthia and is drawn to June.
Diana discovers that Venus is forced to work at the cafe because the owner, Earl Barker, has taken Venus’s pearl tiara, a birthright of magic that grants power to the holder. Diana compels Earl to surrender the tiara and also uncovers a large seashell, the birthright of another mermaid. Venus realizes that the shell belongs to Earl’s wife Betty, who has forgotten her mermaid heritage. Venus returns the shell, but Betty—Holly Brisley, as Young Betty—decides to stay on land because she has a human daughter.
The sisters learn that Mallick has discovered their father’s body and is keeping it in a freezer. They confront him, but Mallick has studied mermaid lore and knows they must grant one wish to anyone who asks for it. He tasks them with finding a chest of sunken treasure. The sisters agree, but devise a plan: Venus will locate the treasure, Diana will retrieve their father’s body, and June asks for Randy’s help, inadvertently revealing her true identity as a mermaid—yet Randy accepts this and is glad. When the sisters deliver the gold, the police raid the warehouse. Checking the freezer, they find the merman’s tail has transformed back into legs as the freezer dries, and Mallick is arrested for murder.
June worries that Randy is drifting away, while Venus confesses that she used her siren ability to make Randy forget recent events to safeguard their mermaid secret. June feels crushed but decides to stay on land in hopes of another chance with Randy. Venus, too, wishes to remain and explore life on land on her own terms. To the surprise of her sisters, Diana chooses to stay on land as well so she can protect her younger siblings as their father would have wanted. The film closes with a sense that the three sisters will continue plotting adventures together in the days to come.
