The Unsent Letter That Unsent a Queen

“That she may have a convenient execution.”

Convenient for whom? The instruction from Elizabeth I’s spymaster, Francis Walsingham, was clear. Mary, Queen of Scots, had been caught red-handed—or red-inked—endorsing the Babington Plot to assassinate her Protestant cousin. The cipher was broken, the letters intercepted, the evidence plain. V. IMPENETRABILI. SCRVTARI. DEVM. NON. OPORTE. We ought not to search into the impenetrable will of God. But searching into the impenetrable will of a monarch? That was Walsingham’s entire profession.

His man in the field, the master forger Thomas Phelippes, had done the work. He’d not only deciphered the letters from Mary to Anthony Babington and his gaggle of Catholic conspirators but, for good measure, added a postscript in perfect imitation of Mary’s hand, asking for the names of the plotters. A neat little trap. Except Phelippes, in his haste to deliver the goods, sent two packages to London. One held the deciphered Babington correspondence. The other, mislabelled, contained a different key entirely. Not the simple substitution cipher used by the plotters, but a far more complex nomenclator—a Vigenère-based system with a codebook of 2,300 symbols—that Mary had been using for years to correspond with Walsingham himself.

Suddenly, the letters painted a very different picture. The original intercept from Mary read not as an endorsement, but as a masterpiece of intelligence-gathering. The plaintext, when decrypted with the *correct* key, wasn’t ‘Let the great enterprise go forward.’ It was ‘Let the great enterprise show its hand.’ A subtle, but rather important, distinction. What Walsingham’s clerks had first read as treasonous plotting was actually a series of meticulously detailed intelligence reports. Mary hadn’t been conspiring to kill Elizabeth; she’d been running the Catholic hotheads as a 20-year sting operation from her various gilded prisons, feeding Walsingham every half-baked plot that crossed her desk, signed ‘Your loyal, if bored, cousin.’ The Babington plot was meant to be the grand finale, the final sweep of radical Catholics before her state-sponsored retirement to a dower house with decent heating.

Panic in Whitehall was immediate and profound. Elizabeth, presented with evidence that she was about to execute her most valuable, if most famously difficult, intelligence asset, flew into a rage that curdled the milk in three counties. The execution warrant, already signed, had to be recalled. But how to explain this to the world? A public pardon would expose the entire counter-espionage operation. An execution would be, to put it mildly, a significant HR blunder. Lord Burghley, a man whose resting face suggested a lifetime of swallowing wasps, proposed a typically understated solution. They would simply declare the whole affair a "misunderstanding," a sort of bureaucratic typo on a national scale. Walsingham, for his part, spent a week drafting a memo on the vital importance of proper key management, written in a cipher so furiously complex that even Phelippes wept when he saw it.

Mary, Queen of Scots, was quietly released and installed in a comfortable manor in Suffolk, under a new and more luxurious form of house arrest. The official story was that she’d had a religious epiphany and renounced her claim to the throne, devoting herself to prayer and embroidery. The unofficial reality was that she spent the next decade running a network of agents from her sewing circle, intercepting Spanish communiqués between bouts of gossip and complaining, in coded letters to Walsingham, that the quality of claret was simply not what it used to be. Anthony Babington and his co-conspirators were still hanged, drawn, and quartered—a necessary piece of security theatre. As Phelippes watched them go, he was heard to mutter, “Well, at least *they* were easy to read.”