England in the sixteenth century is a country in turmoil.
King Henry VIII's Reformation has left England divided between Catholics and Protestants. Most of the those who cling to the Old Faith are content to conform to the laws of the land. But there are some Catholics who wonder where their allegiance lies - to England's heretic queen or to the Pope?
Francis Walsingham has no such doubts. As French ambassador, he witnesses the savagery of the St Bartholomew's Eve massacre, when Catholics slaughtered Protestants in their thousands, and he is determined England will never suffer the same fate.
He begins to create an elaborate network of spies who will keep him informed of Catholic plots against Elizabeth, especially those that have Mary Stuart, Queen of Scots at their heart.
Walsingham's mission is clear: to protect Elizabeth I from her enemies by whatever means necessary!