An assistant detective in Victorian London finally makes his mark.
Thomas Llewelyn, a Welshman with a checkered past, must put aside his plans to marry Jewish widow Rebecca Cowan when the office of his boss, Cyrus Barker, is blown up and Barker winds up in the hospital with a badly broken leg. Llewelyn pores through the ruins, gathering the case files he hopes will pinpoint which of the many criminals Barker has put away may have taken revenge. The dust has hardly settled when Llewelyn is visited by Camille Archer, who claims her husband has gone missing. Suspicious of her story, he follows her, but she gives him the slip. His next surprise visitor is Caleb Barker, Cyrus’ long-lost brother, who works for the Chicago-based Pinkerton Detective Agency. Their meeting begins with a fistfight that leaves each with respect but no love for each other. As Caleb settles into Cyrus' house and spends most of his time working on his brother's caseload, even though Cyrus never asked for his help, Llewelyn doggedly tracks down all the people from their past cases who are still alive to seek revenge. He hires a female detective to track down Camille Archer, who’s now visited Rebecca claiming to be Llewelyn’s sister. The American Embassy wants to get Caleb out of the country, but he evades them and joins Llewelyn as he interviews the suspects in the bombing. Llewelyn’s dreams of marriage are seriously threatened when he’s forced to shoot two people who’ve invaded Rebecca’s home right in front of her and her Aunt Lydia, possibly the only relative who approves of the upcoming nuptials.
As Thomas shifts the focus of his period franchise (Old Scores, 2017, etc.) to a second-fiddle investigator, his hero gathers confidence through every step of a dangerous case.