God tells Moses to take the Israelites in a funky direction so Pharaoh will think they're aimlessly wandering around. God will also harden Pharaoh's heart so that he regrets letting the Israelites go.
Pharaoh's heart is hardened. Again.
Pharaoh decides to go after the Israelites. He rallies 600+ chariots to do the job.
Quick note: here's a great example of the text contradicting itself a little bit. If Israel has 600,000 men, then how does Pharaoh expect to beat them with 600 chariots? The text could be trying to make Pharaoh seem stupid, or it could be the work of two different writers. You decide.
Back in the story, the Israelites see the Egyptians advancing, and start panicking.
When Moses cries out to God, God tells him to stretch out his hand and divide the sea.
God's angel keeps the Egyptian army at bay while the sea opens up to create a passageway. The Israelites pass through the sea on dry land.