Capote's In Cold Blood was based on of the real-life 1959 murders of the Clutter family, who were slaughtered during a robbery gone wrong. The murderers, Richard Hickock and Perry Smith, were eventually hanged after spending five years on death row. Capote effectively transformed a horrific robbery/murder to a page-turning novel. In We Were Killers Once, Masterman portrays the shocking story of Jerry Beaufort, a "third killer." After Jerry's life sentence is commuted, he decides he wants to find Hickock's reported confession, which lays out his role in the killing of the Walker family in Florida.

This murder has long fascinated investigator Brigid Quinn. The case and its connection to the Clutters is the closest she has come to a cold-case obsession. For Brigid and for her husband, Carlo, a Catholic priest turned philosophy professor, the case has morphed into the missing piece of a long-held puzzle. Carlo remembers the Clutters; he was an altar boy at a church not too far from Lansing, the prison where Hickock and Smith were held.

In Capote's book, Perry Smith denied killing the Walters. Capote believed him. But in this new world of 21st-century forensic techniques and cold cases solved by automated fingerprint systems and DNA profiling, Jerry is convinced that someone, someday, will link him to the things he has done. With his paranoid thoughts invaded by death and murder, this ex-con and pseudo-tough-guy researches DNA and rape and a homicide that had occurred 50 years ago. Nobody ever talks about how a killer might suffer while he thinks about being caught.

Back in Arizona, Brigid, Carlo and Gemma-Kate have become accustomed to the worst that life can throw. Brigid wonders how she fits into Carlo's existence and into his memories of Jane, his first wife. Masterman shifts perspective from Brigid to Jerry, which allows her to convert the events of In Cold Blood into a viable plotline. As Jerry retraces the evidence, events lead him to a Florida detective who confides about the whereabouts of Victor Santangelo, a Catholic priest who lives in a secluded Dominican abbey and may hold the key to the whereabouts of Hickock's confession.

From the Kansas Historical Society archives to Santangelo's deathbed confession, Jerry finally feels good to be doing something. When Hickock and Smith were still alive, Jerry lived in fear that they "would rat him out." He stopped worrying when they died, then started worrying again with the advances in "forensic science." Jerry traces the evidence of his involvement to the Tucson area and into Brigid's neighborhood. Initially hooking up with "an old-fashioned broad" called Gloria("not too smart for her own good"), Jerry bides his time, using Achilles, a three-legged dog, to initiate contact with Carlo. Brigid knows there's something off about this new neighbor who is weaseling his way into their lives. Carlo tries to calm her: "just because you got burned once, doesn't mean everyone in Arizona is a villain."

Masterman captures Brigid at her "FBI agent best," the limitations of Carlo's memories of the Clutter family, and Jerry, the monster killer who arrives from a past shadowed by the events of Capote's thriller. When Jerry realizes that Brigid is onto his true identity, he goes after both her and Carlo. Driven by the ferocious need to protect her husband, Brigid does everything that any loving wife would. Masterman's blurred omniscience leads us through a collective rollercoaster of action as she cleverly connects Jerry's beating heart of Hickock and Smith to the brutal slaying of the Clutter family. Detective Meadow was closer than he knew to solving the mystery behind In Cold Blood, the solution no one had dreamed of in nearly 60 years.

As Jerry and Brigid clash in a tense, violent finale, Carlo is forced to fight for his life. Tucson pulsates with tension (a great scene plays out on the road at the base of the Kitt Peak National Observatory). In yet another outing, Masterman's feisty heroine faces many more challenges as a retired FBI officer: the changing nature of crime and an understanding of a marriage that she sometimes thinks she's no longer a part of.