On her seventeenth birthday, Katie discovers a mysterious locket and decides to wear it for good luck. But when her boyfriend Isaac finds out she cheated on him - with their mutual best friend Mitch, no less - he dumps her, leaving her devastated.

And then a miracle happens. The locket burns on Katie's chest, and she feels herself going back two weeks in time, to the night she cheated with Mitch. At first, Katie is delighted to be a better girlfriend to Isaac this time around. But as other aspects of her life become inexplicably altered, she realizes that changing the past may have had a dangerous effect on her present.

Can she make things right before the locket destroys everything - and everyone - she loves?