Codebreakers have cracked a cipher used by Mary, Queen of Scots in a series of letters she wrote while locked up by her cousin Queen Elizabeth I. The 57 decoded letters, written between 1578 and 1584, mark the most significant discovery about Mary Stuart in a century. Most of the correspondences were addressed to Michel de Castelnau de Mauvissiere, the French ambassador to England, and they cover topics ranging from Mary’s poor health and prison conditions to international intrigue involving figures like the queen’s spymaster, Sir Francis Walsingham. It took a team that included a cryptographer, a pianist, and a physicist to crack the coded letters, which were listed as Italian texts in the French National Archive and had long been overlooked. Unexpected French words like “ma liberté” (my liberty), combined with the names of high officials in the British court, tipped historians off to the identity of the letters’ illustrious author.