15th-19th August, 1658: Luka and Emilia must travel to the New Forest to find the Wood tribe, whose charm is a rue flower, the herb of grace. Its power is that of plants and herbs - which can both heal and poison. On the way, the two children tangle with an impoverished widow, the thief-taker Coldham, a highwayman and a witch, and find themselves caught up in a Royalist plot to restore King Charles II to his throne.