Readers not closely familiar with the series opener are also likely to flounder as Haddix closes this duology with continuing underdeveloped plotlines that are made harder to follow by time-travel rules of Byzantine complexity. There’s also some distracting hand-wringing over tangential issues—namely, children’s addiction to smartphones and the pitfalls of AI. Stuck in an increasingly fragmentary reality in which only she notices that characters and items are continually changing, vanishing, and reappearing, seventh grader Maisie meets young people who turn out to be from equally dystopian past and future times—and who themselves repeatedly vanish just as they’re about to tell her something important. Meanwhile, the author gins up both an AI Club to serve as a discussion forum and a strict no-cellphones policy at home and school that hampers Maisie’s efforts to communicate with a mysterious caller who sounds like a strangely unemotional version of her beloved great-aunt. The events move slowly toward a sudden climax that logically resolves little but at least leaves her with new time-traveling friends. Excepting two Black classmates, the central cast presents white.