W tej kontynuacji serialu „H2O – wystarczy kropla” chłopak o imieniu Zac zmienia się w trytona, przez co staje się zagrożeniem dla trzech syren strzegących wyspy Mako.
Ondina is tasked with teaching young mermaids. Rita introduces Weilan, a Chinese mermaid, who will be staying with them. On the way to the Full Moon Ceremony, the girls encounter a water dragon. Weilan warns that there is no way to stop it.
Zac begs Weilan to teach him an Eastern spell that he plans to use against the dragon. Mimmi and Ondina cover for David at the Ocean Cafe, but they encounter problems with Joe and a water ball.
On the eve of the full moon, Mimmi and Ondina defy Veridia's order of watching the hatchlings. Evie becomes suspicious when she spots Zac and Weilan meeting in secret again.