It was with the plague of flies that God choses to set His people apart for the Egyptians. The swarms of flies do not effect Goshen where the Israelites dwell. The Egyptians had to have been perplexed at why their fly god would put swarms upon them and leave the Israelites unaffected. This must have been true for Pharaoh as well, at least temporarily. He agrees to let the people sacrifice but not outside of the land as requested. He again hardens his heart when the plague is gone.