Rosamond Appleton, the impetuous 12-year-old foster daughter of Susanna, Lady Appleton, is horrified when she learns that her French tutor, Madame Louise Poitier, has drowned face down in St. Anne's Well. Unable to accept the crowner's ruling of accidental death, Rosamond calls on Lady Appleton to investigate. Conspiracies and murders surrounding the imprisonment of Mary Stuart, the abdicated queen of Scots, complicate the process, but Lady Appleton and her friend and housekeeper, Jennet Jaffrey, as ever rise to the occasion.