SPOILERS!!!
In France Rollo, known as Jean Danglais, follows the orders of Pierre to contact Mary Stuart so that she can lead the invasion. Now Mary is in Sheffield Castle, and when Alison goes riding, Rollo appears and tells her about Duke de Guise’s plan, Alison says that Mary will doubtlessly agree to help. What Rollo wants is confirmation in writing, so Alison says that there is a servant, Peg, who is dazzled by Mary’s royal person and will be happy to smuggle letters into and out of the castle.
What Rollo does not know is that Ned has a spy in Pierre’s household. It is Alain, Pierre’s stepson, who after his mother died, murdered by Pierre, swore to kill Pierre, but Ned persuaded him to stay with Pierre and inform Ned about what was going on. So Ned knows about the invasion and Mary’s involvement. That is why he goes to Sheffield Castle and bursts into the place. I imagine that this will trigger Mary’s execution on the orders of Elizabeth, who fears for her life and throne.
My Reading Journal 