From New York Times bestselling author Margaret Peterson Haddix comes the first book in a brand-new thrilling series about twins who are on a quest to discover the secrets being kept by their new family. Nick and Eryn's mom is getting remarried, and the twelve-year-old twins are skeptical when she tells them their lives won't change much. Well, yes, they will have to move. And they will have a new stepfather, stepbrother, and stepsister. But Mom tells them not to worry. They won't ever have to meet their stepsiblings. This news puzzles Nick and Eryn, so the twins set out on a mission to find out who these kids are - and why they're being kept hidden.

Nick and Eryn must figure out a way to save the world without sacrificing their newly discovered family in this follow-up to Under Their Skin, which Booklist called “crisp, intriguing, and thought-provoking” in a starred review. In Under Their Skin, twins Nick and Eryn successfully met their new stepsiblings, Ava and Jackson. But in doing so, the twins found themselves on a mission to discover how to prevent humanity from facing certain doom. Now, their two families are joined together to save not only themselves, but everyone—human and robotic. Can they figure out how before it’s too late?