It is the year 1586. England is awash with traitors, plotting to assassinate the Queen and bring about a foreign invasion. The young physician Christoval Alvarez, a refugee from the horrors of the Portuguese Inquisition, is coerced into becoming a code-breaker and agent in Sir Francis Walsingham's secret service. In the race to thwart the plot, who will triumph - the ruthless conspirators or the equally ruthless State?