The year is 1586.
"Tom Walsingham has been tasked with keeping watch over the network of spies recruited by his spymaster cousin, Sir Francis Walsingham. After intercepting a series of letters sent to and from Chartley Castle, where Mary Queen of Scots has been imprisoned, the spies have infiltrated a ring of Catholic plotters. Led by the zealous Anthony Babington, the conspirators plan to murder Queen Elizabeth and replace her with Mary, thus restoring Catholicism to the realm.
With most of the plotters under careful observation, the spies have only to wait for the right moment to have them arrested. However, when one of Tom’s couriers — Isaac Finch — goes missing, it seems that Babington’s conspirators may suspect that they have informers in their midst. Fearing that Finch may have been forced to reveal the spies’ plans, Tom vows to find out who killed him."