Emma Gilbert (Claire Holt), Cleo Sertori (Phoebe Tonkin) and Rikki Chadwick (Cariba Heine) are three fifteen-year-old girls who become stranded on the mysterious Mako Island. When they enter an underground pool of water that leads to the ocean they find themselves bathed in the glow of moonlight from overhead. After being rescued the girls return to their normal lives but soon discover they have changed: seconds after contact with water they transform into mermaids with unique supernatural powers over water. Emma can freeze it, Cleo can control its shape and volume, and Rikki can boil it. The girls enlist Cleo's long-time friend Lewis McCartney (Angus McLaren) to try and determine why this has happened to them and help them avoid being exposed. The series focuses on the girls learning to control their powers and dealing with everyday problems like siblings and accidental contact with water.

The first series features a single primary storyline with three main elements. The girls meet Louise Chatham (Christine Amor), an older woman who reveals that she too was once a mermaid. She helps the girls understand their own situation and warns them of the dangers involved. After a confrontation with Ms. Chatham, Zane Bennett (Burgess Abernethy) gets trapped on her sinking houseboat which forces Emma to rush to his aid. During the rescue, Zane sees her tail and becomes obsessed with finding this "sea monster". Doctor Denman (Lara Cox), a marine biologist, also becomes focused on the mermaids when Lewis accidentally leaves behind a genetic sample of the girls he was working on in her lab. Zane later helps her to capture the girls without realising their identities. When he realises what he has done, Zane works with Lewis to release the girls who trick the scientists into believing they have given up their powers to a lunar eclipse. In reality they have actually only been removed for twelve hours.

2006