While Acton and Doyle, two of Scotland Yard's finest, pursue a self-appointed judge, jury, and executioner, Acton's own questionable methods may prove their undoing. The victims are all criminals who eluded justice--until they ran afoul of an avenger whose modus operandi is a bullet to the back of the head. The key to the vigilante's identity lies in connecting the cold cases to an event that may have triggered retribution after all these years. Meanwhile, Doyle finds herself shadowed by a mysterious figure. After the man steps forward to rescue her from harm, she wonders why he is invested in protecting her. But when she learns he's in contact with Acton's nemesis, she fears she's being used in a plot against her husband. The stakes are high, and both Doyle and Acton must work independently to outwit the players--before their lives are brought crashing down like a house of cards.