Facing the threat of King Philip's Enterprise of England - Spanish invasion and annexation of the country - Sir Francis Walsingham's secret service spreads a network of agents across Europe. After caring for hundreds of maimed and wounded soldiers returning from the fall of Sluys, young physician and code-breaker Christoval Alvarez is sent on two dangerous missions to Amsterdam, where, amongst the friendly Hollanders, treason and treachery lurks. Christoval's ship, sailing home, plays its part in the great sea battle in which the small and inexperienced English navy must confront the most powerful sea force in the world.