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. But what no one knows, except for Jills mom 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—who lives his four days of the cycle in the solitude of Jills room. But Jacks personality has been building over the years since the cycling began. He is growing less and less content with his confinement and his cycles are more frequent. Now Jills question about prom isnt about who she will go with, but who will she be when the big night arrives?