Six days after Mary Queen of Scots gave herself up at Carberry Hill (when she was taken to prison in Loch Leven castle), some letters and poems were found. They had been written by Mary, or so it was claimed, and had been taken from a servant of Bothwell's.
The letters were written in French. The second one (called the Glasgow letter) showed that Mary had been involved in the plan to murder Darnley.
The casket of letters was produced at Mary's trial in 1568. By this time Queen Elizabeth held Mary prisoner in England and allowed the letters to be used as evidence against her.
No-one knows to this day if the letters were genuine or if they were forged by Mary's enemies to make her look guilty.
It would be easy enough to look at the letters today and check the hand-writing and the signatures for ourselves - except that the letters disappeared! They were sent back to Scotland after the trial, and were kept by the Earl of Morton, who found them. After he was executed, the letters were never seen again. No-one knows who took them.
It is possible that Mary's son, James, had them destroyed but no-one will ever know the truth about the letters or what happened to them.