Cody Griffin, a popular high schooler, discovers on his 13th birthday that he is not human but part mermaid. Struggling to fit in, he relies on his friends and family for support. Meanwhile, a fisherman becomes obsessed with finding mermaids after encountering one. Cody excels as a swimmer and forms a bond with a classmate Jess, who helps him with his marine biology project. As Cody's strange symptoms persist, his parents finally consult a doctor who attributes it to puberty. Desperate, Cody seeks Jess' assistance, fearing he may develop a lizard's tail. The story unfolds as Cody navigates his mermaid heritage while facing the challenges of adolescence.