A monstrous regiment of women continues Mary Russell's adventures as a worthy student of the famous detective Sherlock Holmes and as an ever more skilled sleuth in her own right. Looking for respite in London after a stupefying visit from relatives, Mary encounters a friend from Oxford. The young woman introduces Mary to her current enthusiasm, a strange and enigmatic woman named Margery Childe, who leads something called "The New Temple of God." It seems to be a charismatic sect involved in the post-World War I suffrage movement, with a feminist slant on Christianity. Mary is curious about the woman, and intrigued. Is the New Temple a front for something more sinister? When a series of murders claims members of the movement's wealthy young female volunteers and principal contributors, Mary, with Holmes in the background, begins to investigate. Things become more desperate than either of them expected as Mary's search plunges her into the worst danger she has yet faced.