After losing his wife to cancer, Kenneth Hill devotes his life to raising his young daughter Hannah. But when Hannah is killed in a car crash shortly thereafter, Kenneth spirals into addiction and depression.

Kenneth follows his therapist's advice and pens a letter to his daughter as a form of therapy meant to help overcome his grief. Finding this unfulfilling, he mails the letter to "Hannah Hill, Heaven." While such letters, much like those to God or Santa, would usually be marked "undeliverable" and returned to the sender, postal clerk Jennifer Daniels takes the letter home and reads it. Something about this letter feels different.

After reading the letter, she researches the crash and her heart breaks for Kenneth, so she sends a letter back, signing it "Your Princess Hannah." She urges Kenneth to see lightness instead of darkness and tells him that she found "mommy." Her response begins an unlikely pen pal relationship as the well-intentioned Jennifer tries to help lead Kenneth out of darkness; however, she never foresees the letters' true impact.

Author Scott Eisenberg explores grief and mental illness in the dramatic novel Letters to Hannah, an honest look at the effects of losing a child.