Josie Byrne's life is spiraling out of control. And just when she thinks it can't get worse, she starts having dreams about a girl named Jo. Every night at the same time--3:59 a.m. Jo's life is everything Josie wants: she's popular, her parents are happily married, and her boyfriend adores her. It all seems real, but they're just dreams, right? Josie thinks so, until she wakes one night to see a shadowy image of herself in the bedroom mirror. Except it's not her at all--it's Jo.
The two girls are doppelgängers whose universes overlap every twelve hours at 3:59. Fascinated by Jo's perfect world, Josie jumps at the chance to pass through the portal and switch places for a day. But Jo's world is far from perfect, and the stuff of nightmares lurks around every corner. And when Josie finds herself trapped there, her life becomes more dangerous--and more deadly--than she ever imagined.