In MERCILESS, Mercy Gunderson's life seems to be changing for the better. Although she lost her bid for sheriff, Mercy has gained a new boyfriend; the current Eagle River County sheriff Mason Dawson. They have settled down together in domestic bliss at her family ranch, and the former Black Ops sniper has joined the FBI. Mercy has just come home from training at Quantico; and is assigned to the Rapid City, South Dakota bureau under the watchful eye of her trainer, Special Agent Shay Turnbull.

Mercy and the FBI are called to the Sioux reservation after the grisly murder of the young Arlette Shooting Star. Although tribal leader Latimer Elk Thunder and the tribal police don't want the FBI on the reservation, the murdered girl is his niece, so he grudgingly accepts their assistance in investigating the heinous crime. As Mercy begins to delve deeper into the murder, she begins uncovering some unsavory connections, both in the town and on the reservation. Suspects start turning up, some with ties to Mercy. The investigation has not only put her at odds with tribal politics; but also with Mason in his capacity as sheriff, and with several close associates.

After Mercy makes a correlation to the murder with several past suspicious deaths on the reservation, she spends time in the Tribal archives digging up old tribal police records to back up her theories. Personal animosity against Mercy works its way into the investigation, and lies and long held secrets stymie her efforts to resolve the crime. After the third killing in a matter of weeks, Mercy concludes that these grisly murders are the work of a serial killer; tied to the many unsolved crimes she has been uncovering in the archives. A personal event puts a wrinkle into Mercy and Mason's home life when Mason's eleven year old son is sent to live with them by his mother, just as the investigation is heating up. As Mercy delves deeper into the crimes, asking more probing questions and discovering more secrets; she finds that the murders have gotten personal for her, as she has drawn the attention of the killer to herself and her family.

MERCILESS is a very good, interesting addition to the Mercy Gunderson series. Although the story starts a bit slow, the action quickly picks up, leading to an exciting climax in which Mercy brings her specialized military training into play. Local issues and tribal politics add murky layers and twists to the plotline. Mercy makes a few interesting choices along the way; dubious choices that may have a future impact on her personal life, and with her status as a newbie FBI agent as well.