Saint Peter's Fair is a grand festive event, attracting tradesmen from across England and beyond. There is a pause in the civil war racking the country in the summer of 1139, and the fair promises to bring some much needed gaiety to the town of Shrewsbury. Until, that is, the body of a wealthy trader is found in the River Severn.

Was Thomas of Bristol the victim of murderous thieves? And if so, why were his valuables abandoned nearby? Brother Cadfael offers to help the merchant's lovely niece Emma. But while he is seaching for the killer, the man's wares are ransacked and two more men are murdered. Emma almost certainly knows more than she is telling, as others will soon realise.

Cadfael desperately races to save the young girl, knowing that in a country at war with itself, betrayal can come from any direction, and even good intentions can kill.