In the early summer of 1379 in London, Sir John Cranston, Coroner of the city, is trapped into a wager with Signior Gian Galeazzo, Lord of Cremona, who challenges him 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 awful expressions of terror upon their faces.

Realising that his reputation and future wealth now rest upon the solving of this mystery, Cranston seeks the help of his faithful secretarius Brother Athelstan.