Tempest and Tally Jo Trimble are mirror twins—so alike they were almost born the same person—and they've been inseparable since birth. But it's the summer they turn thirteen, and it seems like everyone can tell something is changing between them.

Pa Charlie, whose traveling carnival is the best part of every summer, is watching them closer than ever. Digger, who sneaks poor kids onto the carnival rides for free and smiles faster than anyone, seems to be fixing for a fight. Even Mama is acting different, refusing to travel with the carnival this year even though her own twin, who she hasn't seen since childhood, will be there.

And Tally and Tempest are the most different of all. There's a strangeness between them, a thickness to the air, an unseen push and pull, and it's getting stronger. It starts as a feeling, but soon it's sputtering and sparking, hurling them backwards, threatening to explode.

When Tally learns that she and Tempest may not be the first twins in their family to be separated by whatever this force is, she realizes she'll have to find a way to stop it—or she might lose not only her sister, but everyone she loves.