An outcast at school, Chao Lung-chieh takes solace in his grandfather's company. However, after the two have a fight, things go from bad to worse.
Lung-chieh tries to deal with his fallout with Hsu Tsung-chien, but a man with a mermaid tail won’t stop following him. An accident changes everything.
Lung-chieh introduces the merman as his cousin Lee Jen-yu. Meanwhile, Jen-yu reveals the prophecy he must fulfill. Chiang-chiang gets suspicious.
Accidentally elected as the class representative, Lung-chieh develops a bond with Jen-yu. But Jen-yu soon messes things up for him and Chou Pi-yu.
With his friendship with Lung-chieh on the rocks, Jen-yu discovers his true purpose for coming to land and devises a plan to make Lung-chieh happy.
Jen-yu learns of the cause of the rift between Lung-chieh and Tsung-chien. Guilt drives him to take action against Ta-pao, which leads to consequences.
Lung-chien begrudgingly keeps his best friend’s secret, while Jen-yu confronts Chiang-chiang about what he’s been hiding. Ren-yu’s plan backfires again.
Secrets are out after Chiang-chiang overhears a conversation. As rain starts to fall, everyone’s in for a shock after a series of missed encounters.
After his birthday, Lung-chieh grows closer to Pi-yu, but his happiness is short-lived as the class has to take part in a school swimming competition.
Lung-chieh’s confidence grows as he learns to swim with his friends' help. Chiang-chiang learns a shocking fact and hatches an ominous scheme.
It’s the day of the swimming competition, and Lung-chieh finally stands up for himself. Meanwhile, Chiang-chiang and his dad have other plans.
Lung-chieh achieves a hard-earned victory, but celebrations descend into chaos as he must save his merman friend from being hunted by humans.
Lung-chieh's troubles are far from over as a surprise betrayal leaves Jen-yu in the hands of the fishermen. An unexpected individual joins the rescue.
Lung-chieh puts everything on the line for Jen-yu in the final showdown between the students and the fishermen.
