Content Warning: This novel discusses eugenics, forced institutionalization, racism, and child sexual abuse. It also uses outdated terminology for discussing mental health and disabilities, which is reproduced in quotation in this guide.

Mary Engle is the narrator and protagonist of The Foundling. The narrative is presented from her first-person point of view and traces her growth from a meek and lonely orphan girl to an empowered woman, friend, and wife. She comes from a poor family, and her father and uncle were both criminals. Her mother died when she was young, and her father put her into St. Catherine’s Orphan Asylum in Scranton, Pennsylvania. She stays there until she is 12 years old and is subjected to sexual abuse from her Uncle Teddy, whose death she feels responsible for. She is best friends with Lillian Faust at the orphanage, whom she meets again at Nettleton where Lillian is an inmate. By reuniting with Lillian, Mary comes to see that her worldview is limited, and her journey to self-discovery parallels her work helping Lillian escape Nettleton. She also learns more about the world through Bertie, a nurse at Nettleton, and Jake, a Jewish journalist whom she dates and eventually marries.