Isabel Dalhousie, Edinburgh philosopher and curious observer of the behaviour of her fellow man, is approached by a friend at a local boarding school that is planning to appoint a new headmaster; an anonymous letter has arrived suggesting that one of the shortlisted candidates has a compromising past. But which one is it? Isabel is once again drawn into an investigation, and finds herself exploring dilemmas of human weakness and forgiveness. She turns to her fiance Jamie for advice, but he too appears to have something to hide.