Abby Barnes had a plan. The Plan. She'd go to Northwestern, major in journalism, and land a job at a national newspaper, all before she turned twenty-two. But one tiny choice—taking a drama class her senior year of high school—changed all that. Now, on the eve of her eighteenth birthday, Abby is stuck on a Hollywood movie set, miles from where she wants to be, wishing she could rewind her life. The next morning, she's in a dorm room at Yale, with no memory of how she got there. Overnight, it's as if her past has been rewritten.

With the help of Caitlin, her science-savvy BFF, Abby discovers that this new reality is the result of a cosmic collision of parallel universes that has Abby living an alternate version of her life. And not only that: Abby's life changes every time her parallel self makes a new choice. Meanwhile, her parallel is living out Abby's senior year of high school and falling for someone Abby's never even met.

As she struggles to navigate her ever-shifting existence, Abby must let go of the Plan and learn to focus on the present, without losing sight of who she is, the boy who might just be her soul mate, and the destiny that's finally within reach.