Amy Samuels (Christine) is a preteen girl who loves to swim and is constantly in a Sibling Rivalry with her older sister Tess (Bridget). After Amy mysteriously sprouts a mermaid tail and powers, she wishes for a friend to rely on, but she also wants to protect her secret. Her new friend, Brenna, becomes a mermaid as well. Another new friend, Kelsey, becomes jealous of their friendship, discovers their secret, and soon shares it. They learn they each have a different power: Amy has water (manipulation of water), Brenna has heat (fire, boiling), and Kelsey has ice/freezing. One day while swimming, they meet Blaire (Tessa), whose mother was a mermaid and abandoned her and her father to live in the ocean. The three mermaid girls struggle with keeping their secret, having problems with their powers, and interacting with Blaire.
This series provides examples of:
- Adults Are Useless: Borders on There Are No Adults. The only adults appear for only a scene or two. They're Amy and Tess' mother, Brenna's mom, Amy's aunt and Blaire's dad. Bridget briefly plays Kelsey's mother in Power Hour, whose only purpose is to comfort Kelsey from a nightmare.
- Brought Down to Normal: In Scarce Scales the three girls lose their tails briefly.
- "Do It Yourself" Theme Tune: Christine wrote and recorded one with her cousin, and sings a cover of Swimming by Florence + The Machine (in Season 5, Emma sings it with her.)
- Playing with Fire: Amy could boil water at first, but Brenna receives this power later on. Her power ranges from hot air to literal fire to boiling water.
- Parental Abandonment: Although she has the excuse of being a mermaid, Terra left Blaire and her dad on her daughter's first birthday.