Most likely, the plagues never did happen, but may have been a story passed down from from a previous times, woven into the biblican history of the region. The hardening of the Pharaoh's heart is probably a parable to show how unbelievers are evil and even after seeing the signs, do not heed to them.

Of course, we have to go by the assumption that god controls everything and in this case, god controls the Pharaoh as well.