The film begins by presenting a beautiful yet isolated beach, where a young girl gazes out at the ocean, her parents nowhere in view. Years later, we meet Mia, a fifteen-year-old girl who relocates to a new town because of her father's job. On her first day at her new school, Mia is captivated by a group of rebellious classmates, led by the bold and daring Gianna. She introduces herself to the group and quickly becomes part of their circle, spending time at Gianna's home.
As part of their friendship initiation, the girls create an online dating profile for Mia, questioning her sexual history. Eager to impress, Mia fabricates stories about her experiences and reveals her interest in older men. During a risky game, Gianna encourages Mia to choke her, leading to an exhilarating moment for Gianna. When Mia attempts the same with Gianna, she has an unsettling experience and envisions herself submerged underwater. Disturbed, she rushes home.
Back home, Mia exhibits impulsive behavior by catching a few tropical fish from her parents' aquarium and consuming them whole. When her mother, Gabriela asks about the missing fish, Mia lies, claiming they were flushed. Furthermore, as she adjusts to her new environment, she experiences significant changes in her body, including the onset of menstruation and physical abnormalities like webbed toes and unexplained bruises. Driven by her suspicions about being adopted, Mia feels isolated, especially when she cannot find any evidence of her mother's pregnancy.
One day, after being dropped off by her father Michael, she skips school to visit a healthcare clinic, desperate for answers about her condition. Initially dismissed, the doctor later determines that her webbed toes are likely a lifelong condition requiring surgery. After a blood test, Mia sneaks back to school feeling overwhelmed.
Mia receives a message from an older man via her dating profile and arranges to meet him for sex. Outside the hotel, her friends wait nervously. During the encounter, as things heat up, the man becomes alarmed by the bruises on her legs and backs off, leading to a tense confrontation. Disappointed, Mia returns to her friends, who ridicule her for not going through with it.
Mia's escapades continue as she visits Roberto, a friend of Gianna’s, where they engage in a casual hookup. The party atmosphere escalates when Mia consumes alcohol and ecstasy with Gianna and their companions. Unfortunately, a night of debauchery leads Mia to feel shamed by Roberto, who dismisses her as someone who "gets around" too much. After a rough comedown, Mia finds herself vomiting in a restroom while Gianna's friends express concern for her increasingly battered appearance.
With tensions mounting at home due to her rebellious actions, Michael insists that Mia return to therapy. One day at school, during a fish dissection lab, Mia finds herself unable to resist and consumes the dissection subject, shocking her classmates. Tensions escalate when word spreads about her bruises.
After declining an invitation from her parents to attend a wedding, Mia is drawn to a dock party where, in a dramatic twist, she saves Gianna from drowning. In a state of shock following her heroic act, she heads home only to realize her legs are transforming into fish scales. When Gianna discovers Mia's condition, she is horrified; however, Mia lashes out, unable to comprehend her rapidly changing body.
Eventually, Mia's transformation intensifies, leading to her legs merging into a single fin. In desperation, she attempts to harm herself but instead loses consciousness. Awakening to the shocking realization that she has transformed into a mermaid, Mia plunges into a bath full of water, creating chaos in her home.
In the bustling climax, Gianna arrives, torn between fear and concern. She helps Mia escape to the coast where a poignant farewell occurs. Mia shares her last thoughts with her mother over the phone, expressing her longing for her family. In a heartfelt moment, Mia and Gianna share a bittersweet goodbye before Mia returns to the ocean, fully embracing her new mermaid identity.
