Alright, so this plague was introduced even before the first plague. Let us go back to Chapter 4 of Exodus (I quote the 1917 edition of the JPS as I have little time and therefore please excuse the archaic terminology):
21 And the LORD said unto Moses: 'When thou goest back into Egypt, see that thou do before Pharaoh all the wonders which I have put in thy hand; but I will harden his heart, and he will not let the people go. 22 And thou shalt say unto Pharaoh: Thus saith the LORD: Israel is My son, My first-born. 23 And I have said unto thee: Let My son go, that he may serve Me; and thou hast refused to let him go. Behold, I will slay thy son, thy first-born.'Okay, but I find a disconnect between verses 22 and 23. I don't know if any commentators pick up on this but I have always seen verse 23, in the second person singular, as directed at Moses not Pharaoh. What happens immediately after is an angel trying to kill either Moses or his son and is only let go when Tziporrah gives him a bris. Why else would it need to say "and I have said to you"?
Why the plagues? The following may seem heretical but I want to place this in a historic context. It appears to me that many of these plagues are directed not only at Pharaoh and Egypt but at their gods. Judaism was not at this point monotheistic. The second commandment is that "You shall have no other gods before Me" not "You shall have no other gods". You could have other gods, but the Lord is the head of the pantheon, the supreme God. Numbers 21:14 mentions a missing book called "The Book of the Wars of the Lord". The Psalm for Tuesday mentions God standing in the court of El denouncing the other gods assembled. "Who is like you among the gods, Lord?". Where is that? It is the translation of Mi Chamocha. There is apparently celestial struggle and that the plagues are directed at destroying the gods of Egypt. As I mentioned last week, the plagues were introduced against the four elements, all deified in Egypt. The Nile was sacred because it brought life to the Two Lands, so God killed it, hence the blood. He killed all of the cattle on three occasions (I don't know how they had any left after Dever/Cattle death, but the ones of those who didn't fear God died in Barad/Hail and then the firstborn beasts died during Makat Bechorot/Death of the Firstborn.
What is the point of all this. This is God who cannot be attacked. To precursor the tenth plague God had the Israelites take one of Egypt's top deities, the lamb, and tie them to their houses, while the shocked Egyptians looked on. After four days, they slaughtered the lambs and smeared their blood on their doors. The Yiddish language has the most appropriate word for this: Chutzpah! Wow, the gumption and cojones of the Jews to schmear the lamb's blood on their doors and barbecue them right in front of their oppressors (possibly during the plague of darkness when the Egyptians couldn't move). This is the only plague that has an observance surrounding it. It, to this day, is the most observed Jewish custom. More than fasting on Yom Kippur, more than lighting Chanukkah candles, more than 98% of Jews have some sort of Passover Seder. This is such an important time of year that the mitzvah is given here to make Aviv (in our modern calendar, Nisan) the First of Months, something which lasted probably until the hiatus in the Jewish monarchy. The rabbis suggest that the Torah should have started here.
While death was all around them they were able to have a quiet meal. The family unit and the community are so important, to comfort one another in the face of death and to remain strong.
Though the Egyptian firstborns die, God has a special role for the Jewish firstborn. First they are the original Kohanim, the priests (later supplanted by Aaron and his descendants) but they always must be redeemed from this Holy task with which they are commanded. The first of all Kosher animals (as well as donkeys for some reason) must be redeemed as well.
Two of the four texts in the tefillin are found at the conclusion of this Torah portion. Two of the Four Sons' questions (from the Seder) are also found in this portion.
"It came to pass at midnight", but now the Israeloids (they are technically not -ites yet until they are in the atmosphere, right? are packed, their loins girded, and their matzah not leavening, and they will leave first thing in the morning.