Fifteen-year-old Aya Fuse, an Extra, just wants to lay low, so when she discovers the secret lives of the Sly Girls, she wants to report their story, but she knows that would propel her into a celebrity status she's not prepared for.

In an alternate 1914 Europe, fifteen-year-old Austrian Prince Alek, on the run from the Clanker Powers who are attempting to take over the globe using mechanical machinery, forms an uneasy alliance with Deryn who, disguised as a boy to join the British Air Service, is learning to fly genetically-engineered beasts.

Just before their sixteenth birthdays, when they will will be transformed into beauties whose only job is to have a great time, Tally's best friend runs away and Tally must find her and turn her in, or never become pretty at all.