Sixteen-year-old Winnie Schulde has always seen splits—the moment when two possible outcomes diverge, one in her universe and one in another. Multiverse theory, Winnie knows, is all too real, though she has never been anything but an observer of its implications—a secret she keeps hidden from just about everyone, as she knows the uses to which it might be put in the midst of a raging WWII. In this darkly thrilling novel that blends science and war with love and loss, some actions just can’t be undone.