In the wake of the 1588 destruction of the Spanish Armada, English Catholics launched an ingenious counterespionage effort to undermine the Tudor government's anti-Catholic machinations. This Jesuit-connected network secretly transmitted intelligence to Brussels, Antwerp, Madrid and Rome. Its central figure was William Sterrell, a brilliant Oxford philosopher. Sterrell moved at the highest levels of government, working for the ill-fated Earl of Essex and for the powerful 4th Earl of Worcester, secret sponsor of the Jesuits. Partnered with Sterrell was Phelippes the Decipherer, once aide to Sir Francis Walsingham, now turned Jesuit agent. Queen Elizabeth long maintained spies and provocateurs among English Catholic exiles. Walsingham, her principal secretary, used treachery-including the execution of Mary Queen of Scots-to foster plots against the queen to justify harsh measures against Catholics. His agents were involved in the Jesuit Mission of 1580-1581, the Throckmorton, Parry and Babington plots and the defeat of the Armada. His successor, Sir Robert Cecil, continued to infiltrate Catholic exiles and stir dissension. Sterrell's agents nimbly countered Cecilian intrigues until the Gunpowder Plot of 1605. This is the story of Sterrell's secret network-undetected for 400 years-brought to life in vivid detail, based on close examination of hundreds of original letters and documents never before transcribed or published.