In 1586 the catholic conspirator Anthony Babington plotted against the protestant Queen Elisabeth I to advance Mary Stuart to the English throne. Mary Stuart was being held captive. All her correspondence was spied upon and read by Walsingham, the chief minister of Queen Elisabeth. So Babington and Mary encrypted their letters about the conspiracy to prevent non-authorized eyes to read the contents. They used a mix of a cipher and a code, which was a quite common way of encrypting diplomatic correspondence in those days.
What they did not know, was that Walsingham employed a gifted mathematician and linguist, Thomas Phelippes, who managed to break the conspirators' code. Walsingham and Elisabeth could now read everything about the plot. As could be expected, it would be quite unfavorable for the Queen's health if the plan would be brought into practice.
They could just have arrested the two conspirators, but Walsingham decided otherwise and waited. He wanted to arrest everyone involved in the plot. Because the other conspirators were unknown, Walsingham and his cryptographer decided to insert a request into one of the letters from Mary to Babington. In the code that Phelippes cracked, they encoded a request to Babington to mention all his fellow conspirators to Mary and added this request to a genuine letter.