Hulme wrote the book based partly upon the experiences of her friend, Marie Louise Habets of the Sisters of Charity of Jesus and Mary, a Belgian nurse and an ex-nun (it is often erroneously stated that the book was based upon Hulme's own experiences). The lead character of the book, Sister Luke (real name Gabrielle Van Der Mal) finds her faith tested in Africa where she finds herself at odds with headstrong Dr. Fortunati, operator of a remote Congo hospital, with whom she gradually builds respect, and again during World War II when she is ordered not to take sides. Ultimately, Sister Luke is forced to decide whether to remain in the convent or return to the outside world.

Gabrielle/Sister Luke is stretched between her desire to be faithful to the rule of her congregation and her desire to be a nurse. As a nun she must remove all vestiges of "Gabrielle Van Der Mal" and sublimate herself into the devoted bride of Christ. As a nun there is no room for her personal desires and aspirations. Ultimately, the conflict between her devotion to The Church and the nursing profession, juxtaposed with her passionate Belgian patriotism and her love of her father (killed by Nazi fighter planes while treating wounded) bring her to an impasse, which serves as the dénoument of the novel.