An unlikely shipboard romance occurs in 1840 between a wealthy young Mormon convert from England, Mary Rose Ashley, traveling with her family from England to America, and Gabriel MacKay, one of the builders and designers of the new Cunard line who is evaluating the clipper ship’s performance. Married on board, the newlyweds make their way to the new Mormon settlement in Nauvoo, Illinois and are just settling in when Prophet Joseph Smith receives a revelation from God about polygamy. As one of their close friends from the voyage is suddenly widowed during anti-Mormon riots, Gabriel announces he will marry the beautiful Bronwyn and raise her baby as his own. Mary Rose loves Bronwyn like a sister, but cannot imagine sharing her husband. Assuming that the relationship will remain platonic, she agrees, and Bronwyn and Mary Rose begin to come to terms with what plural marriage truly entails. Is this really what God wants for them?

Ensnared in a religion she increasingly questions, Bronwyn faces an impossible choice. When recently widowed Bronwyn becomes the second wife to Gabriel MacKay, she promises her new sister wife and best friend, Mary Rose, that the marriage will remain platonic. But she finds herself falling in love with the man who has promised to keep her safe—even as her heart struggles with her promise to her friend. When Gabriel marries a third wife, and as the practice of polygamy increases and the age of brides decreases, Bronwyn begins to question the righteousness of this new religion. Even as the Saints journey to Salt Lake Valley, their new promised land, Bronwyn and Mary Rose plot their escape. But when they are suspected of treachery by Brigham Young's secret army—whose mission it is to punish apostates—the two sister wives face the most difficult decision of their lives, one that can mean life or death for those they love.