The plagues seem arbitrary and strange until we realize that each plague correlates with an Egyptian god. In other words, God is working up the hierarchy of the Egyptian gods, showing that His strength is greater because His strength is real.
This simple graphic is by Kevin Ohlin:
Here's another overview of the Exodus, in an artistic whiteboard form:
Whiteboard Bible 3 - The Exodus from World Outreach Church on Vimeo.