On 1 December 1581, the same day that Edmund Campion and two other Catholic priests were torn apart by the executioner at Tyburn, a prisoner wrote from the Tower of London to Elizabeth I’s principal secretary, Sir Francis Walsing-ham. Praising Walsingham for his ability to forecast conspiracies against the state while they were still forming in the minds of plotters, John Hart proposed a deal with the very man who had put him in prison. In exchange for his freedom, Hart would infiltrate the English Catholic exiles on the Continent and gain the confidence of their spiritual leader, Cardinal Allen, in order to discover the ‘very secrets of his whole heart’. On the surface it was an attractive offer, since Hart was himself a Catholic priest: several of his kind defected to the establishment during Elizabeth I’s reign, apostates among the company of saints and martyrs who have been formally recognised by the Vatican since the 1880s. On this occasion, Walsingham chose to decline. Hart’s sudden release from custody might have looked suspect, and the community of Catholics both at home and abroad had learned to be alert to the danger of spies. But, as Stephen Alford’s deep and convincing new study of the Elizabethan security services confirms, Catholic priests continued to end up on both sides of the political divide thrown up by the English Reformation. As the Babington plotters and Mary, Queen of Scots, all learned to their cost, the impact of a Catholic clergyman changing sides could be devastating.