In early 1302 a violent serial killer lurks in the city of London, slitting the throats of prostitutes.And when Lady Somerville, one of the Sisters of St Martha, is murdered in the same barbaric fashion, her death is closely followed by that of Father Benedict in suspicious circumstances. Edward of England turns to his trusted master clerk, Hugh Corbett, to reveal the identity of the bloodthirsty assassin. Joining Corbett on his mission are his devious manservant Ranulf and his faithful horseman Maltote.

In the dark, fetid streets of the city and in the desolate abbey grounds, they encounter danger and deceit at every turn. Only Ragwort, the mad beggar, has seen the killer strike, and the one clue that Corbett has to help him is Lady Somerville's cryptic message: 'Calcullus non facit monachum ' - the cowl does not make the monk.