Complex sentence: The plot was cynically manipulated by Walsingham in order to bring about his primary objective of the destruction of Mary Queen of Scots and when Mary rather foolishly gave her consent to the assassination plot by replying to a letter sent to her by Babington her days were numbered.
Split into simpler sentences: The plot was cynically manipulated by Walsingham in order to bring about his primary objective: the downfall of Mary Queen of Scots. When Mary rather foolishly gave her consent to the plot by replying to a letter sent to her by Babington, her days were numbered.