Peng Shepherd's The Book of M is a work of fantasy fiction set in a future world. The events of the story are kickstarted when, one day, a man in India loses his shadow. He is the first of many, as people's shadows start disappearing all around the world. As people lose their shadows, they also start losing their memories. The condition spreads, spreading panic along with it.

Our main characters, Orlando "Ory" Zhang and his wife, Max, have also lost their shadows. Max lost hers first, and Orlando is now determined to save her memories for as long as he can. The pair have a code between them, where they ask each other a question of personal relevance to their relationship. If either of them forget the answer to this, then it would be a sign that their memories are fading. Max is worried that once her memories are gone, she might become dangerous and harm her husband. As such, she leaves without telling him.

Ory begins looking for her as soon as he discovers this. Along the way, we meet other characters, such as "the amnesiac," a man who lost his memories in the aftermath of a car accident. He is later called "The One who Gathers," and his shadow never disappears. We also meet Mahnaz "Naz" Ahmadi, an Iranian girl and professional archer. All the events in the quest to find Max seem to conspire to draw Ory to New Orleans.

Max is also traveling there in order to find the source of a rumored cure and has many adventures of her own on her journey. Ory has been tracking her by using a series of recordings she left behind. The ending of the book is very strange and ambiguous, where, instead of reuniting with Max, Ory meets Ursula, who has Max's memories and her personality—presumably having been bestowed them by "The One Who Gathers" through Max's tape recorder.