Queen Elizabeth I's Secret Service
During Elizabeth I's reign, an extensive network of spies and code crackers uncovered scores of assassination plans, including the famous Babington Plot. Unwilling to entertain rivals to the throne, Elizabeth held her cousin Mary, Queen of Scots, under house arrest for 18 years. During this time, there were many plots to free Mary and install her on the throne. Francis Walsingham, Elizabeth's spy master, intercepted a message between Mary and Catholic crusader Anthony Babington about plans to assassinate Elizabeth. The code was a simple substitution cipher and was easily cracked. Walsingham then used its contents to trick the conspirators into revealing all the participants, leading to Mary's execution.