Milly, her mum and her glamorous babushka head to the Nutcracker for a Christmas treat – but instead their evening ends in a bomb scare disaster. A dangerous trickster who calls himself 'the Mouse King' is playing a deadly game with Swan House, the ballet school for spies. As Milly navigates her second term, she starts to suspect there is more to the new Head of Ballet and his perfect daughter than meets the eye... but will she let jealousy cloud her judgement?