This is it. The big one. God and the destroyer kill every firstborn Egyptian, from the prince right on down to the firstborn calves. Exodus tells us in 12:30 that, "There was not a house without someone dead." Grisly.
The Egyptians, including Pharaoh, have had enough and tell the Israelites to get lost ASAP.
The Israelites leave so quickly that their bread doesn't rise in the oven, but they do save enough time to "plunder" the Egyptians of all their gold and silver (12:36).
Exodus tells us that 600,000 men, plus some unmentioned number of women and children, left Egypt at the end of 430 years in that land. Time out: 600,000?! The world population at the time was less than 50 million people, so we're talking at least 2% of the entire planet's population. That's a lot of people.
Archaeologically, there is no evidence for any kind of migration of this size. That's not to say that it didn't happen, though. After all, the ancient Egyptians didn't like to record their defeats, so even if some form of a migration happened on a smaller scale, the scribes would not have written it down.
Back to the story. In 12:43-50, God tells the Israelites how to include other peoples in the Passover ceremony—they must be circumcised.
P.S. In the beginning of Chapter 12, God gives instructions for the Passover ceremony that marks his liberation of the Israelites from Egypt. Basically, you kill a lamb, paint its blood on the outside of your door, and eat unleavened bread (bread that doesn't rise). The blood tells God not to destroy an Israelite's house, and the bread signifies that the Israelites had to leave quickly. So why did we talk about this first piece of the chapter last? Because this stuff is more "rules and regulations"—the story doesn't really start until 12:21.