Summer, 1379. Sir John Cranston, coroner of the city of London, is trapped into a wager with Signor Gian Galeazzo, Lord of Cremona, when challenged to resolve a certain murder mystery within two weeks. Men have been found dead in the scarlet chamber of one of Cremona's manors. They have no mark upon them; they have neither drunk nor eaten poison; there are no secret passageways or entrances to the room. And they all have an awful expression of terror upon their faces. Realising his reputation and future wealth rest upon the solving of this mystery, Cranston seeks the help of his faithful secretarius, Brother Athelstan.