This book explores the careers of three French Missionary nuns in the first half of the 19th century. It explores the ways that these women were able to use their traditional roles in a heavily patriarchal organization, ie the Catholic Church, to go places and do things that lay women, especially unmarried women, could never do.
(Think of protestant Jane Eyre- "you have to marry me and go risk your life in India" "Hmm....couldn't I just do *one* of those things???" "NO- do what I say!")
It's also about the shift in convent focus and life after religious orders were re-authorized under Napoleon. Basically the old, contemplative model gave way to a new generation of nuns who wanted to get out and be useful- and in this case acts as agents of imperialism.