Exodus 8:20-24

And the Lord said unto Moses, Rise up early in the morning, and stand before Pharaoh, lo, he cometh forth to the water; and say unto him, Thus saith the Lord, Let my people go, that they may serve me. Else, if thou wilt not let my people go, behold, I will send swarms of flies upon thee, and upon thy servants, and upon thy people, and into they houses: and the houses of the Egyptians shall be full of swarms of flies, and also the ground whereon they are. And I will sever in that day the land of goshen, in which my people dwell, that no swarms of flies shall be there; to the end thou mayest know that I am the Lord in the midst of the earth. And I will put a division between my people and thy people: tomorrow shall this sign be. And the Lord did so; and there came a grievous swarm of flies into the house of Pharaoh, and into his servants houses, and into all the land of Egypt: the land was corrupted by reason of the swarm of flies. EXODUS 8:20-24

The Lord spoke to Moses - get up early and speak to Pharaoh. Tell him to let my people go so that they may serve me. God's objective is not only to set the people free from slavery but to set them free so that they can serve Him. Many time we may seek deliverance and healing from God without recognizing that our purpose is to serve Him. We seek gifts and blessings then seek to serve self rather than our Father God creator of heaven and earth. God is sending His messenger to the one who is holding His people captive. God could have just set Israel free right from the beginning. He didn't need to send Moses and He certainly doesn't need to ask Pharaoh's permission to receive what already belongs to Him. Why do you suppose that God chose this way to set His people free?

God let Pharaoh know what would happen if he did not let the people go. You would think by now that Pharaoh would realize that God kept His Word. This time God said that plague would not come upon His people but would come upon Pharaoh, the people and the land of Egypt. God is in total control He not only can send a plague but decides to whom and where it will be sent.
God controls the timing of the plague. Fear not God is in control and if we are living in a time of captivity it is God who will decide what can and can not come upon His children.