The following version of the book was used to create this study guide: Henry, Patti Callahan. Becoming Mrs. Lewis. Thomas Nelson, Nashville, TN. 2020. Kindle AZW file.

Joy Davidman grows up in a strict household with demanding parents. She considers herself an atheist and marries Bill Gresham. The two are writers and parents of two young sons, Davy and Douglas. Bill often spends time drinking and carousing with other women, leaving Joy worrying at home. One night, she falls to her knees in prayer and realizes God is real. From that moment, she desperately seeks answers to spiritual questions. She learns that an English author named C.S. Lewis had a similar experience. Joy writes to him in the hope of finding some of those answers. Lewis, known to friends as Jack, begins to help Joy navigate her concerns. Joy becomes determined to show Jack her true self. Through their letters, they become friends. Since Joy has health issues, her family doctor warns Bill that Joy might not recover from another crisis. When Bill makes no secret he is having sex with other women, Joy moves out of their bedroom. When her cousin Renee seeks a place to hide from her own abusive husband, Joy welcomes her into her own home. As Joy's physical and emotional health continue to fail, Renee encourages her to visit England. Joy decides to go, noting that she can seek much-needed medical and dental care while there. She leaves Davy and Douglas with Bill, Renee, and Renee's two children.

In England, Joy is captivated by the land, architecture, and people. She sees doctors and dentists, all at no charge, and does research for her next book project while writing a series of articles she also hopes to publish as a book. Though she feels better physically and emotionally, she desperately misses Davy and Douglas. She spends months in England and comes to love Jack and his brother Warnie as her family. Just as Joy books passage home, Bill writes that he and Renee are in love. He urges Joy to return, find someone of her own in New York, and allow Bill and Renee to marry.

Back in America, Joy feels trapped by lack of money, continuing to live in the house with Bill, Renee, and the four children. She decides she is going to move to England, though she has no idea how she will get the money to do so. Renee moves to Florida, believing Bill will soon join her there. Within days, Bill writes to Renee that he is staying with Joy. He begs Joy for another chance to make their marriage work. Joy is tired of the chaos and refuses. Bill goes through various jobs, including traveling with a circus. Joy saves every penny for the move. She sells the house and takes the boys to England.

Davy and Douglas do not immediately settle in. Jack helps Joy pay for their nearby boarding school. Joy can visit every week. Davy and Douglas are happiest when the three spend holidays at Jack's estate, the Kilns. Joy is completely in love with Jack, but he continues to insist on remaining just friends, even after Bill's attorney sends Joy the divorce decree. Bill and Renee marry immediately. Jack and Joy marry in a secret civil ceremony, but they continue to live apart. Officials from the Church of England refuse to marry them because Joy is divorced.

Joy continues to have health issues, including pain that is diagnosed as rheumatism. One day, she collapses with a broken leg. The doctors diagnose cancer that began as a lump in her breast and has now spread to the bones. Joy endures painful surgeries and treatments, ending with the doctors saying she has very little time to live. A priest decides God would approve of their marriage and performs it in the hospital room. He also prays for miraculous healing. Joy goes home to the Kilns to die, and Jack prays to take her pain. Soon, doctors say Joy's bones are growing back, though Jack is losing bone mass. Jack becomes dependent on a back brace. They have three years together, traveling and loving each other fully before Joy dies. Jack dies three years later. Douglas and Davy remain in England.