How can Dorothy Martin clear an elderly woman of murder, when she can't remember if she did it or not?

American Anglophile Dorothy Martin and her husband, retired chief constable Alan Nesbit, are enjoying a pleasant, albeit rain-soaked, stay in the historic university city of Durham with their old friend David Tregarth. Castle tours and cathedral services fill their days as they wander up and down the steep, cobbled lanes of the medieval city. But the holiday comes to an abrupt halt when David's elderly great-aunt is accused of the murder of a fellow patient at her nursing home. How could a frail dementia sufferer commit such a terrible deed? Dorothy is determined to prove Aunt Amanda's innocence, and launches into an unofficial investigation. But as she and David navigate their way amongst reckless students, inefficient police work and witnesses who have lost the ability to remember, the investigation soon becomes as twisty, slippery and treacherous as the damp cobblestones outside . . .