Anonymous commented on my near oversight of not posting a Sefas Emes on his yahrzeit, so let me try to correct that!
There are a number of interesting points in the opening to parshas Bo:
1) Hashem tells Moshe to go to Pharoah and warn him, but the text interestingly does not contain any reference to the makkah which will be brought (see Ramban).
2) Why does the text use the word "bo" instead of "leich"?
3) When Pharoah gives him he admits "chatasi la'Hashem v'lachem" -- this is the only occurance of an admission of sin not just against G-d but against the Jewish people as well.
R' Simcha Bunim of Peshischa (Kol Simcha) explains that in fact Moshe was not told in advance what the upcoming makkah would be. Therefore, the Torah does not say "leich", implying shlichus, but rather uses the term "bo". Moshe was given free reign to choose what makkah to inflict on the Egyptians and based on his understanding of midah k'neged midah arranged for the plague of arbeh. Hashem wished to show that the purpose of the makkos stemmed from his love for the Jewish people and his responsiveness to their needs and not just for the abstract goal of punishing the Egyptians. When Pharoah begs for mercy he must confess that it is not just G-d who he has wronged and is subservient to, but also the Jewish people, who have within their power to ellicit punishments from G-d upon those who wrong them.
The Imrei Emes quotes the Sefas Emes as explaining that this makkah represents the revelation of torah sheba'al peh in the process of the makkos. One means of Hashem communicating with the world is the direct revelation of His will that occurred at Har Sinai and to the Nevi'im. However, there is another means of Hashem's will becoming known, and that is the process of torah sheba'al peh -- what the chachamei hador intuit and infer to be the ratzon Hashem, what the chachamei hador declare to be the ratzon Hashem, is the ratzon Hashem. Moshe did not need a revelation of the details of the makkah of arbeh in advance; it was his intuition of the ratzon Hashem that lef him to understand that this is the ratzon Hashem.
It is no coincidence that this process is revealed during arbeh, the eighth makkah in the series of makkos. In the list of sefiros and midos the lower seven midos all represent action, while the upper three represent knowledge -- chochma, binah, da'at. Each of the makkos was a revelation of some new aspect of Hashem's relationship and control over the world. This eighth makkah of arbeh is the start of the revelation of chochmas haTorah, and therefore it contains the germ of torah sheba'al peh, the ability to use chochma to reveal the ratzon Hashem.
The Maharal frequently writes that the number seven represents the order of the natural world, e.g. seven days in a week, but the number eight represents transcendence beyond those boundaries. The midos beyond seven are the vehicles that allow us to transcend our own physical limitations, as the mind has no boundaries as to what is can conceive of or imagine.
The Sefas Emes explains that this is why the Torah here stresses not just the goal of "v'yedatem ki ani Hashem" as a reason for the makkos, but also stresses the need "lma'an saper b'oznei bincha", the transmission of the story from generation to generation. Torah sheba'al peh is the mesorah of klal yisrael, the blueprint of Judaism that is not reducable to a chiddush or shita that can be formulated in a book or in an explicit command, but must be given over as a way thinking and living that allows us to see the ratzon Hashem using our chochma so that generations after Sinai we still feel Hashem speaking to us and making his will known.