In 'The Thirteenth Year', the protagonist Cody Griffin becomes a merman. His transformation begins around his thirteenth birthday, when strange abilities and physical changes reveal his connection to the sea.
Cody is adopted and initially lives as an ordinary teenager, but he develops extraordinary powers including exceptional swimming speed, the ability to communicate with fish, long underwater breath-holding, and electrical shocks. Scales and fins gradually appear as his transformation progresses. His friend Jess helps investigate the mystery and concludes that Cody is becoming a merman.
The film’s central secret is that Cody’s biological mother is a mermaid. She left him with his adoptive family as a baby to protect him, and eventually returns to help him understand his changing identity. The title refers to the milestone birthday at which Cody’s inherited nature begins to emerge; the answer is not “mermaid,” because Cody is male.