It's 1284 and Edward I is battling a traitorous movement founded by the late Simon de Montfort, the rebel who lost his life at the Battle of Evesham in 1258. The Pentangle, the movement's underground society whose members are known to practice the black arts, is thought to be behind the apparent suicide of Lawrence Duket, one of the King's loyal subjects, in revenge for Duket's murder of one of their supporters. The King, deeply suspicious of the affair, orders his wily Chancellor, Burnell, to look into the matter.

Burnell chooses a sharp and clever clerk from the Court of King's Bench, Hugh Corbett, to conduct the investigation. Corbett - together with his manservant, Ranulf, late of Newgate - is swiftly drawn into the tangled politics and dark and dangerous underworld of medieval London.