Gabrielle "Gabby" Van Der Mal (Audrey Hepburn), a headstrong young Belgian girl and daughter of a prominent Belgian surgeon enters a convent with the ambition of becoming a nursing sister in the Belgian Congo. However, from the moment she enters the convent, she struggles to adapt to cloistered life and the convents strict rules, in particular the Grand Silence and the vow of obedience. When she becomes a novice, Gabby is renamed Sister Luke and is submitted to a series of tests which are designed to root out her faults and destroy her pride, which she struggles with throughout the film. Friendships are forbidden in the convent, and this ultimately leads to one of the other novices (Patricia Bosworth) leaving the convent before taking her first vows; this particular novice states that even though Rev. Mother Emmanuel (Dame Edith Evans) said "You can easily cheat us, your Sisters, but you cannot cheat yourself or God", she feels it would be cheating God to go on. Sister Luke eventually takes her first vows upon which she is sent to the School of Tropical Medicine. When the Rev. Mother Marcella (Ruth White) asks Sister Luke to intentionally fail her exam to show humility, she finds herself unable to do so. Even though she passes, Sister Luke is left disappointed when she finds she is being assigned to, not the Congo, but to a mental hospital. While there, an especially violent patient who thinks she's the Archangel (Colleen Dewhurst) tricks Sister Luke into opening the cell door then attacks her, but Sister Luke barely manages to escape from her. Despite her disobedience, she is eventually allowed to take her final vows and is sent off to the Congo. Shortly after her arrival, she learns that she will be assisting Dr. Fortunati (Peter Finch), a brilliant surgeon whom Sister Luke is warned is "an unbeliever". As time goes by, Sister Luke finds herself under increasing stress, ultimately falling ill with tuberculosis. Not long after she recovers, Sister Luke is crushed to learn she's being sent back to Belgium as the only nurse qualified to accompany a VIP who has become mentally unstable. After a while, Mother Emmanuel gives Sister Luke a new assignment; as Sister Luke can't return to the Congo with talk of war, she is instead made assistant in surgery at a local hospital. At this point, Sister Luke's struggle with obedience becomes impossible to sustain. Then, she receives a letter from her brother, Pierre, telling her that the Nazis have killed their father; this is the final blow for Sister Luke who, with some difficulty, asks for and is granted a dispensation which releases her from all her vows. In the movies final scene, Sister Luke changes into civillian clothing and leaves the convent through a back door. She walks to the end of the alley and after a moment of hesitation, she heads off to the right, bringing the movie to an end.
Out of all her films, Audrey Hepburn said this one was her personal favorite.
Ingrid Bergman turned down the role of Sister Luke, claiming she was too old, and suggested Warner Bros. offer Audrey Hepburn the role.