While exploring the mysterious Island of Mako, three quite ordinary teenage girls fall under a spell that changes their lives. From now on, whenever water splashes any one of them, they are transformed into mermaids! Endless situations and missions draw the three friends into the incredible underwater world where they make new friends: Bernie the hermit crab, Zita the inquisitive eel, Teddy the kid turtle, Sue the mystical manta ray and Carlotta the diva-jellyfish – and have endless fun and wonderful, enchanting adventures under the sea.
Rikki Chadwick is a rebel who'll try anything and not even think of the risks. She takes on adolescence like a mountaineer takes on a peak: directly, with force and determination. She always goes where others dare not, always does what others don't do, and sets herself apart from the herd, from society. For her, becoming a mermaid is an unbelievable stroke of luck. Now she's really different! And she's planning to make the most of it. Zane is very interested in Rikki, and likes her toughness. Rikki is not indifferent to Zane, but neither is she blind to his vanity and bullying, and gives him a very hard time when she wants to.
Emma Gilbert is logical, mature and a perfectionist. Sometimes she gets obsessive about what she's doing and can even forget her friends for her studies or her interests. But she never forgets to look after her little brother Elliot, and when decisions need to be made, she's right there to take the lead. Becoming a mermaid exacts a heavy toll on Emma, since she was a dedicated swimmer before her fateful expedition to Mako Island. Now it's impossible to plunge into a pool without turning into a mermaid... and so Emma no longer goes near one. Though she misses swimming terribly, she pretends to have gotten over it.
Cleo Sertori is timid, intuitive and wants to become a vet for marine animals when she grows up. Her weakness is that she's sometimes too shy, even to the point of blocking her ability to respond. She laughs and cries easily. For Cleo, becoming a mermaid is like a pendulum that swings between being “the most totally amazing thing that could ever have possibly happened” and sometimes it's a horrible punishment she didn't do anything to deserve. It depends on the day and what's happening around her. Lewis has a big crush on her and she likes Lewis too, but – as she says to herself – as a brother.
Lewis McCartney is a little genius, the kind of guy who surfs the school program effortlessly and wraps up his homework before everyone else. As a result, he comes across as someone who makes no effort. His career dream is to become an inventor and researcher in robotics. Although he's top of the class, he's not a nerd and the butt of every joke of his classmates: quite the opposite. He's a good-looking boy, he knows how to send back a joke, and he's not afraid of Zane, although he finds him exasperating. Lewis is the only person outside the trio of our heroes who knows about the spell over them.
Bernie the hermit crab is a little fellow who can be characterized by his expression “It always happens to me!” He has a plastic toy car for a shell and a kind of Alladin's cave full of treasures for a home. He sees himself as the General of the underwater world and has authority over the whole underwater community (except the Vandals). Bernie is the essential link with the girls and, through them, with the world of humans. When something goes wrong underwater, he comes and finds the girls. Let's say they all have a common duty to protect and restore the harmony and safety of the Bay.