On a boat in his youth, Chi Ekin Cheng Yee-Kin nearly drowns, unable to swim, but washes up on a beach claiming rescue by a "big fish." The incident leaves a lasting mark: even years later, he carries a fear of the water, shaping his life as he grows into a young teacher.
Years later, [Ekin Cheng Yee-Kin]–or rather Chi–brings a new calm and energy to a local high school, where his good looks and youthful charm draw students to him. Among the students, the flirtatious Kiki Teresa Mak Ka-Kei, nicknamed "Princess," tests his professionalism, positioning herself against the returning jealousy of her on-again, off-again boyfriend, Kenji Takeshi Kaneshiro. Chi navigates the delicate line between being approachable and maintaining boundaries, all while the school ecosystem buzzes with rumors and attention.
One day, Chi leads his class on a field trip to the docks. An altercation leaves him shoved into the water, where he sinks unconscious. He is rescued by an orange-tailed mermaid—Christy Chung Lai-Tai—the same creature who once saved him on that beach years ago. She emerges with a magical pearl, which she releases from her stomach to revive him. In a moment of fate, Chi swallows the pearl, waking as though he had merely drifted ashore. The mermaid, unable to return home without her pearl, realizes she must find Chi and thus assumes a human guise with legs as long as she avoids getting wet.
The mermaid, now walking among humans as Siu-may, tracks Chi to his school, where she is mistaken for a worker’s niece and befriends Kiki, slipping into the student body as a new pupil. Siu-may can walk on legs, but her unusual need to bathe periodically to "recharge" reveals her true nature. A bathroom encounter in the girls’ room exposes her secret to Miss Yuen Kingdom Yuen King-Tan, the stern teacher who becomes fixated on catching the so-called "big fish." Miss Yuen enlists the school’s principal Kent Cheng Jak-Si and others in a pursuit that pits tradition against a love that can bridge two very different worlds. Kenji, who first senses there’s more to Siu-may than meets the eye, promises to help keep her hidden, and a tentative friendship forms between them.
Chi and Siu-may grow closer as the story unfolds. Chi discovers his swimming ability has returned, a change he attributes to the pearl within him. The two test their limits together in the school pool, even agreeing to a blindfolded swim when Siu-may claims she lacks a swimsuit. As their bond deepens, Siu-may contemplates the pearl’s retrieval by using a kiss, and she agrees to a dinner date filled with clams and quiet, shared intimacy. Yet the moment is delicate: she forgets to summon the pearl, caught up in her feelings for Chi.
On Chi’s birthday, he confesses his feelings for Siu-may, and she hesitates, admitting that she is a "fish" who cannot stay. She allows herself to stand out in the rain and transforms into her mermaid form before a frantic intrusion by Miss Yuen and her cohort, who seize Siu-may and haul her away to the principal’s swimming pool for captivity.
A daring trio—Chi, Kenji, and Kiki—stages a rescue. They infiltrate the pool area, defeat the surrounding watch, and free Siu-may, returning her to the docks where she can reclaim the pearl and, perhaps, a future with Chi. The morning after their escape, Siu-may possesses her pearl once more and prepares to return home. The couple declare their devotion openly, and Siu-may dives back into the sea, transforming and swimming away, leaving Chi with a hopeful but bittersweet goodbye.
The pursuit by Miss Yuen, the principal, and their allies continues as Chi follows a path that blurs the lines between human and myth. He encounters a wedding bridesmaid who bears a startling resemblance to Siu-may, triggering renewed questions about identity and memory. The principal even deploys a hose to try and provoke a transformation, but the ploy fails, leaving the adults confused and on the run. Kenji and Kiki step in to clear the path as new possibilities arise.
In the end, the living memory of Siu-may lingers in Chi’s heart. When he meets the bridesmaid again, a quiet recognition passes between them—she clearly remembers nothing at first, yet a shared moment over clams rekindles a spark. Siu-may’s echo remains with Chi, and as the two embrace, the screen closes on a weft of love that has endured despite the barriers of sea and shore, leaving open the possibility that destiny has more in store for them beyond the surface.
