On her 17th birthday, Katie discovers a 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, Kate is delighted to be a better girlfriend to Isaac this time...