As far as anyone at her high school knows, Jill McTeague is an average smart girl trying to get her dream date to ask her to the prom.
What no one knows, except for Jill's mum and dad, is that for the four days Jill is out of school each month, she is not Jill at all. She is Jack, a genuine boy - complete with all the parts. Jack lives his four days per month in the solitude of Jill's room. But his personality has been building since the cycling began. He is less and less content with his confinement and his cycles are becoming more frequent. Now Jill's question about the prom isn't who she'll go with, but who she'll be when the big night arrives.