Milly yearns to dance like her ballerina mum—but during the biggest performance of her life, she messes up and her mum disappears. Six months on, Milly receives an unexpected invitation to join the Swan House School of Ballet. Thrilled, Milly accepts, only to find that Swan House is no ordinary ballet school: it's a ballet school for spies. Can her new skills help her discover the truth about her mother's vanishing act?