A mad prophet predicts the doom of an ancient city, prompting the only one who believes him to rise to leadership and lead a group of other deviants out of the doomed city in search of a new place to live. Along the way they find another city, living a lie as their members die slowly. They escape and find a place to live, but nearby is the realm of a dictator who oppresses his people and is set to make war on the new colony. The leader of the outcasts decides to liberate the free-thinkers among the oppressed people and have them join their new utopian society.
Okay, now, as you probably know, all of the above characters are rabbits. The story Watership Down is incredible: it's totally rabbit-oriented, having them all act exactly like rabbits would, except of course giving them the ability to talk to each other. In order to do this they usually speak English, so that we can understand them, but the rabbits also have their own language, which is used when there is no English word for what they're trying to say. A lot of the characters' names are in rabbit talk (even if it's usually translated), so Fiver is actually Hrair-roo and Bigwig is actually Thlayli.
Also, the rabbits have their own religion. Several myths are told at various parts of the book about the king of the rabbits, El-Ahrairah, the Prince of a Thousand Enemies.
This book is about rabbits, yes, but even if you don't normally like rabbits you'll end up enjoying this book for the adventure and action in it. Yes, action: when I saw the cartoon I thought it was disturbing to see rabbits tearing the living hell out of each other, but apparently that's what they do.
Bottom line: this book kicks ass.