Piper has lived her life as a normal teenage girl but when she discovers that she is an alien, her whole world begins to unravel. Her Earthly parents are afraid of her and her true parents died long ago on her home planet. Scientists discover her secret and try to perform unusual experiments on her. They give her the nickname Watergirl once they discover she has gills. Her closest ally is her twin brother, Ky (Fireboy). They discover their planet's past and the horrible truth: a race of aliens known as the Yairganders destroyed their planet as well as many others. Every planet sent a hero and Piper finds that she is the prophesized and reluctant leader of them all. She must try to stop the Yairganders from doing to Earth what they did her planet as well as find the other aliens and get them all back to their home planets safely.