Even though your question does not mention the context of these sneaky French spies, I am going to make an educated guess that you are talking aboutKing Lear.
At the beginning of Act 3, scene 1, Kent relays some valuable information to one of Lear's knights. Kent knows that French spies have moved onto the island with French soldiers, almost ready to strike. Meanwhile, the spies pretend to be servants in the homes at Albany and Cornwall.
The presence of the spies in England are important, because they are feeding information to the king of France about King Lear's condition and the actions of Goneril and Regan.