A mysterious assassin prowls the narrow alleyways of London. February 1304, and a succession of brutal murders shocks London as it comes to terms with the fall from power of Walter Evesham, Chief Justice in the Court of King's Bench. Accused of corruption, Evesham has sought sanctuary to atone for his sins. When Evesham's clerk is cruelly murdered, and then Evesham himself is discovered dead in his abbey cell, it appears that the Mysterium, a cunning killer brought to justice by Evesham, has returned to wreak havoc. Sir Hugh Corbett is ordered to investigate. Has the Mysterium returned or is another killer imitating his brutal methods? As Corbett traces the ancient sins that hold the key to discovering the killer's identity he must face his most cunning foe yet.