As the travellers gather in the Tabard Inn at the start of a pilgrimage to pray before the blessed bones of St Thomas à Becket in Canterbury, they agree eagerly to mine host Harry's suggestion of amusing themselves on each day of their journey with one tale and each evening with another - but the latter to be of mystery, terror and murder.

The Knight begins that evening: his tale opens with the destruction of a sinister cult at its stronghold in the wilds of Oxfordshire by Sir Hugo Mortimer during the reign of William the Conqueror, and then moves to Oxford some two hundred years later where strange crimes and terrible murders are being committed. The authorities seem powerless but Lady Constance, Abbess of the Convent of St Anne's, believes the murders are connected with the legends of the cult and she petitions the King for help.

As the murders continue unabated, special commissioner Sir Godfrey Evesden and royal clerk Alexander McBain uncover clues that lead to a macabre world sect, which worships the dark lord. But they can find no solution to a series of increasingly baffling questions and matters are not helped by the growing rift between Sir Godfrey and McBain for the hand and favour of the fair Lady Emily.