Emilia and Luka have secured two of the six ancient lucky charms, an old gold coin and a small silver horse. They must now travel to the new forest to find the Woods tribe, whose charm is a rue flower, the herb of grace. it's power is that of plants and herbs, which can both heal and poison. On the way the children tangle with an impoverished widow, a highway man and a witch, and find themselves caught up in a Royalist plot to restore King Charles II to his throne.