Sixteen-year-old Kathy is on the run from a convent orphanage. She must reach Dublin to find her best friend but she's woken up in an Irish Traveller camp, miles from anywhere. Her only chance of getting to the city is with their help, but for that, she needs to become one of them.

Kathy has until the next full moon to find a talent and prove herself worthy. She enlists the help of a gifted young fiddler named Heath. But the others have become suspicious of Kathy, made worse by the disaster that follows in her wake. Only the matriarch of the clan seems to believe in Kathy's potential.

Time is running out for Kathy as she races across Ireland, along the way encountering fairy forts, changelings and tinkers. And while she tries their gifts of fortune telling and palmistry, nothing seems to work for her - until she discovers the Book of Moons.