One evening in 1588, just weeks after the defeat of the Spanish Armada, two young men landed in secret on a beach in Norfolk, England. They were Jesuit priests, Englishmen, and their aim was to achieve by force of argument what the Armada had failed to do by force of arms: return England to the Catholic Church.

Eighteen years later their mission would be shattered by the actions of the Gunpowder Plotters -- a small group of terrorists who famously tried to destroy the Houses of Parliament -- for the Jesuits were accused of having designed "that most horrid and hellish...