February 1304, and London is in crisis. A succession of brutal murders shocks the city as it comes to terms with the fall from power of Walter Evesham, Chief Justice in the Court of the King's Bench. Accused of bribery and corruption, Evesham has sought sanctuary to atone for his sins.

When Evesham is discovered dead in his cell at the Abbey of Sion though, 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 the murder. 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 murderer's identity he must face his most cunning foe yet.