Eighteen-year-old Serenity Ashdown has a brilliant mind: she counts, calculates, and analyzes everything, all the time. Awkward. When her father suddenly disappears, Serenity follows his trail to a parallel dimension. The feds on the other side claim to want to help her go home, if she helps them reconstruct the right codes for the portal between worlds. But it's soon clear they want something more: a gateway for invasion, because this version of Earth is dying.

When Serenity learns that her alter-ego was killed in battle, she assumes "the other Serenity's" identity and uses her unique abilities to lead the resistance in a mortal fight against the tyrannical super-government that is poised to invade her universe. Serenity has no idea how to be someone she's not, but she has to try--or she may not have a home to return to.