Home at Greenoak in Hampshire, Texas, transplant Pru Parke joins a society to establish a medieval herb garden near the twelfth-century Hospital of St. Cross in Winchester. But when a flamboyant supporter of the garden who promised a million-pound donation is found hanged, Pru finds herself drawn into more than the study of medieval gardens.

A dangerous herbal concoction made from an ancient recipe was found near the body. Had the poison been taken willingly or under duress? Was it suicide or murder? Murder, police say. Detective Inspector Christopher Pearse—Pru's husband, partner in crime-solving, and the only one legally allowed to do so—is called in on the case by Detective Superintendent John Upstone, who was Christopher's first "guv" more than twenty years earlier.

Murder is a fact, but motives involve emotions—deep grudges, resentment, jealousy, revenge. Who are the likely suspects? What will happen to that promised huge donation? Pru and Christopher investigate. Like walking two different paths in the garden, she follows her heart and he follows the evidence, but they end up with the same answer: someone else is in danger.