After three years away, seventeen-year-old Brodie McKellon has returned to live with her eccentric grandmother above the last remaining Dead Letter Office - the place letters go when no one is left to claim them. But with her reputation as a troublemaker, things don't stay quiet for long, and Brodie soon reunites with her childhood best friend, Elliot, and her old sidekick-turned-nemesis, Levi, to investigate an unsolved mystery: the unclaimed letters of a group of friends who seemed to vanish without a trace nearly twenty years ago. As Brodie, Elliot and Levi are drawn into the riddle of the dead letter writers, they discover that the past is never truly past, and that old troubles sometimes resurface, with unexpected consequences.