Emotionally scarred after battling cancer, amateur historian Cora Tozzi is troubled by religious doubts. She immerses herself in the life of a convent in her home town, Lemont, Illinois, agreeing to coauthor a book about the history of the religious order. Soon she discovers her problems are only beginning. She befriends a harmonica-playing young nun who is guarding a secret, and when Cora's husband, Cisco, becomes seriously ill, she prays for help to a mysterious visitor from heaven. Soon Cora and her friends plunge into a desperate quest to solve a mystery from the life of a woman destined for sainthood, racing against time in the belief that solving the mystery will restore Cisco's health. The quest takes them to 1886 San Antonio, Texas.