Roger and Humfrey Jolland and their kinfolk, the Medleys, are faced with the Spanish Armada.

As the Spanish Armada sails to England, Humfrey and Roger Jolland are the guests of their kinsmen, the Medleys of Mantlemass. Humfrey, bold and thoughtless, rides off secretly, hoping to see a great battle. He does not return, but Roger - a strange boy, not strong but possessing a "seeing eye" - is convinced that his brother is not dead. Roger is left at Mantlemass to be lovingly cared for by his cousin, Ursula Medley. Growing up, he becomes absorbed in the affairs of Mantlemass and of the ironworking at Plashets.

As war threatens England, the greed of traitors taints the wealden forest. Horses vanish from their stables, plans for weapons turn up missing from the forges, and smoke curls from the chimney of a cotaage long empty. Treason is lurking within the wood of Mantlemass. Ursula wonders who might betray England and turn against her?