H2O: Just Add Water

Plot
Rikki Chadwick, Emma Gilbert, and Cleo Sertori are three sixteen-year-old Australian girls who find themselves stranded on the mysterious Mako Island. They eventually find themselves in a pool under a dormant volcano just as a full moon passes overhead, bathing the pool in light. The girls are rescued and brought back to shore where they return to their normal lives, but soon discover their lives have become anything but normal. The girls find out that seconds after contact with water, they transform into mermaids. After further experimentation, the girls also discover they have supernatural powers over water. The trio enlist the help of Cleo's friend Lewis McCartney to help them keep their secret.

Everyday situations, such as bathing and rain, become tricky as the girls struggle to cope with their newfound abilities, which have many advantages and disadvantages, while also trying to keep them a secret from everyone else, including their families. They soon adapt to their new abilities and lifestyles.

Series two introduces Charlotte Watsford, a rival to Cleo after taking Lewis as her boyfriend. She eventually gains mermaid powers as well and becomes the main antagonist of series two. However, she eventually loses both her powers and Lewis.

Series three sees the departure of Emma, who has left to travel the world with her parents, and the arrival of Bella Hartley, who has been a mermaid since the age of nine. Rikki and Cleo eventually become friends with Bella, but are soon beset by a mysterious tentacle of water with a connection to Mako Island. A new boy, Will Benjamin, also arrives and become friends with the trio when he discovers they're mermaids. The girls eventually learn that Earth is in the path of a comet that could destroy the planet. Using their powers, they manage to stop the comet from colliding with Earth.