"What, Hal Hempseed?" replied the mercer. "Why, you may remember he was a sort of a gentleman, and would meddle in state matters, and so he got into the mire about the Duke of Norfolk's affair these two or three years since, fled the country with a pursuivant's warrant at his heels, and has never since been heard of."
The Duke of Norfolk's affair spoken of in Walter Scott's "Kenilworth" is the Ridolfi Plot, which sought to eliminate Elizabeth I from the throne in favor of Catholic Mary Queen of Scots. Roberto di Ridolfi, who is credited with hatching the plot, was a Florentine banker with ties to William Cecil, among others. Mary was to marry cousin Thomas Howard, the 4th Duke of Norfolk, as part of the plan.
The conspiracy fell apart due primarily to the work of Admiral John Hawkins, who worked as a counter-espionage agent, and learned of Ridolfi's plan from Spain's Ambassador to England. On January 16, 1572, Thomas Howard was tried for treason for his role in the conspiracy. He was found guilty, and executed.