In the novel "His Fair Assassin: Grave Mercy," the once abused Ismae Rienne not only learns how to love and follow her heart, but also realizes that her gift of death can be used mercifully, not just as a means of vengeance. Set in the duchy of Brittany in the late 1400's, the novel addresses the themes of religion, the roles of women and the questionable nature of blind obedience. Ismae must weigh her own knowledge and instincts against information being sent to her by her convent in order to save Brittany and its young duchess as well as the man whom she has learned to love.

Because she survived an attempt to abort her, it has been declared that Ismae is the daughter of Death. Because of this identity, she is hated by her father and abused by the people of her community. At seventeen, she is sold to a man in an arranged marriage. When her husband realizes her lineage, he beats her badly and threatens to turn her over to the church. Ismae is saved by the hedge priest who married the two and spirited away to the convent of St. Mortain where she learns the art of assassination.

In the third assignment given to her by the convent, Ismae is placed with Gavriel Duval, half-brother of the duchess of Brittany, to determine if he is a traitor. During her time with Duval, Ismae falls in love with him and realizes that he is not a traitor even though others are trying to make it appear that he is. Although it goes against the orders the convent has sent to her, Ismae teams up to help Duval ensure that his sister, Anne, is crowned as the duchess of France. The two also try to arrange an acceptable marriage for the young duchess that will insuring that she has a decent husband and that the interests of Brittany are protected.

Realizing that it is not Duval who is behind the treacherous acts being committed against Anne and her court, Ismae seeks out the identity of this traitor. She learns that it is Chancellor Crunard, who also serves as liaison for the convent, who is sowing the seeds of distrust against Duval. In addition, Crunard has also been poisoning Duval in an attempt to kill him. In a twist at the end of the novel, Ismae realizes that she has the ability to save Duval from the poison.