After Mary tells the story of Isabel Stanhope, a secret Catholic devotee of Mary Ward in Protestant England of the early 1600's. Rich in research, McMahon traces Isabel's story from England to Rome, and is highly successful in creating an atmosphere of whispering conspiracy and the loss of faith that accompanies Isabel's eventual disappointment with Ward. Yet the book faces the same problem that often attends historical novels attempting to recount a time when there was a different idiom: that is, how does one accurately relate the mood of past times using an idiom that the modern reader can understand? McMahon is often unsuccessful in her attempt to balance the past with the present, and thus behind many well written scenes lies language which jars the reader into the present. Sometimes McMahon is so intent to connect with her audience that she resorts to tired clichés: 'She was not after all a minor member of his vast flock, but Isabel, Mistress Stanhope, whose presence cut through his priesthood like a knife through thick butter'.