That is, until this summer. The twins are traveling with Pa Charlie's carnival just like always, but there's a new distance between them. Tempest is so caught up in her own ideas, she doesn't seem to have space left in her life for Tally. And, more than that, Tally's started to notice there's something between them. Something real, growing with the phases of the moon, pushing them apart. Sparking, sputtering, wild. Dangerous.

With the full moon approaching, Tally knows it's up to her to find out what's going on--and to beat it. If she can't, she might just lose her sister. Forever.

For fans of Savvy and A Snicker of Magic, this is a spellbinding story of friendship and family--a poignant ode to both what's worth holding on to and what we have to let go.