Short answers, pulled from the story.
Mary Stuart entered the world on the 8th of December 1542 at Linlithgow Palace. She was the only legitimate child of King James V to survive him and inherited the Scottish throne as an infant.
Mary escaped from Lochleven Castle with the aid of George Douglas and raised an army of 6,000 men. She met Moray's forces at the Battle of Langside on the 13th of May, was defeated, and fled south to seek asylum from Elizabeth I of England.
Mary was executed after being implicated in the Babington Plot which involved the attempted assassination of Elizabeth I. She was convicted of treason on the 25th of October 1586 and Elizabeth signed the death warrant on the 1st of February 1587.
Mary's husband was Lord Darnley who was found dead in the garden of Kirk o Field after an explosion on the night of the 9th to the 10th of February 1567. Bothwell was generally believed to be guilty of Darnley's assassination by the end of February 1567.
The casket letters were eight unsigned letters purportedly from Mary to Bothwell found in a silver-gilt casket less than 12 inches long. They are widely believed to be crucial as to whether Mary shared the guilt for Darnley's murder although their authenticity remains a source of controversy among historians.