London, 1940. When nurse Elsie offers to send a reassuring letter to the family of a patient, she has an idea. She begins a book of last letters: messages to be sent on to wounded soldiers' loved ones should the very worst come to pass, so that no one is left without a final goodbye. But one message will change Elsie's life forever. When a patient makes a devastating request, can Elsie find the strength to do the unthinkable? London, present day. Stephanie has a lot of people she'd like to speak to: her estranged brother, to whom her last words were in anger; her nan, whose dementia means she is only occasionally lucid enough to talk. When she discovers a book of wartime letters, Stephanie realises the importance of our final words - and uncovers the story of a secret love, a desperate choice, and the unimaginable courage of the woman behind it all.