Read Exodus 7.

God spoke to Moses, reaffirming his role as a prophet to Pharaoh, with Aaron serving as his spokesman. God warned Moses that he would harden Pharaoh's heart so that, despite the signs and wonders performed, Pharaoh would not listen — and, through this, God's power would be demonstrated and his people ultimately delivered from Egypt.

Moses and Aaron obeyed and approached Pharaoh. Aaron threw down his staff, and it became a serpent. Pharaoh, unimpressed, summoned his own wise men and sorcerers, who replicated the feat through their dark arts — each of their staffs also becoming serpents. However, Aaron's staff swallowed up all of theirs. Even so, Pharaoh's heart remained hardened, just as God had said, and he refused to listen.

God then instructed Moses to meet Pharaoh at the bank of the Nile the following morning. Moses was to take his staff and declare that the God of Israel had sent him, warning that Pharaoh's continued refusal to release the Israelites would bring consequences. Aaron was to strike the water of the Nile with his staff as a sign of God's authority and power.

Aaron struck the Nile, and all of its water turned to blood. The fish in the river died, the water became foul, and the Egyptians could not drink from it. The plague extended throughout all of Egypt, affecting water held in vessels of wood and stone. Yet again, Pharaoh's magicians were able to replicate this sign, themselves turning water to blood, which gave Pharaoh cause to dismiss the miracle. He turned and went back to his palace, unmoved. The Egyptians, meanwhile, were forced to dig along the banks of the Nile in search of drinkable water, as the river itself had become undrinkable.