Exodus Unveiled # 3.  The Breaking of Egypt.  Overview & Insights on the Parshat Bo with Rabbi Jonathan Feldman on Jewish Matters Podcast. 

The last three plagues finally break Pharoh and Egypt and the Jewish people are sent free.  Before they go they are told to have a feast of lamb and matza, and to put the blood on the door lintel.  Three times they are told that in the future they are to re-enact this night and have a seder, and tell their children the story.