When Mariah White catches her husband, Colin White, with another woman for the second time in their marriage, he files for divorce and gives her full custody of their child, Faith.
After the divorce, Mariah falls into a deep depression, while Faith develops an imaginary friend called her "Guard". When she starts quoting parts of the Bible, which she has never read, her mother takes her to see a therapist, fearing for her sanity. The therapist finds that Faith may in fact be seeing God.
News of this reaches Ian Fletcher, a "teleatheist" who is traveling around the country debunking "miracles" that feature God. Ian shows up at the Whites' house, where he has a confrontation with Millie Epstein, Faith's grandmother. This leads to her later having a heart attack and being taken to hospital, where she is declared dead. About an hour later Faith kisses her good-bye and thus apparently raises her back to life. This increases the attention of the general public focused on her, with complete strangers and media representative surrounding the house, making it impossible for the Whites to continue with their normal lives.
She performs a few other miracles, including healing an AIDS baby and showing what appear to be stigmata. Colin, hearing of the commotion surrounding his daughter, sues Mariah for custody of Faith. Mariah and Faith flee to Kansas City, where they are confronted by Ian, who is visiting his autistic brother, Michael. They end up staying together after Ian cheats Mariah into believing he will not betray them and give them accommodation. After a confrontation with Faith trying to "heal" Michael, Ian blows up. However, Mariah and Ian appear to have fallen in love.
After hearing from her mother that Colin is suing her, they fly back. Meanwhile, Faith is admitted to the hospital again and her mother is prevented from seeing her by the court, at which point Faith's symptoms begin to worsen, eventually leaving her near death. Upon being reunited with her mother her health increases drastically, and in the end, Mariah is awarded custody, the holes in Faith's hands disappear, and her "guard" goes away. Mariah and Ian begin a romantic relationship during the novel and they and Faith eventually live together as a family. But at the very end of the story, it remains unclear whether the guard is really forever gone or is still with Faith.