Delaney Moss should have died. She was supposed to die.

She never expected, after years of waiting, to receive a heart transplant. Now, seventeen-year-old Delaney doesn't know what to do with her life. So Delaney latches onto the one thing that gives her meaning: learning as much as she can about the person whose heart she inherited.

After some sleuthing, Delaney concludes her heart donor was a girl her age, Mia, who died tragically one town over. Delaney tells herself that attending a post-mortem birthday celebration for Mia will help her fully know who Mia was. And she's sure that after the memorial, she will be able to move on with her life. But then she meets Clayton: a boy who makes her new heart race; a boy whose grief and outlook resonate with her, and a boy who, it turns out, was Mia's best friend. Suddenly Delaney is more drawn to Mia than ever...