Helen, yes - getting the Mahabharata into a nutshell is quite tough.
Ravi has said what it is about - rivalry between two sets of cousins for the throne of Hastinapur. I thought the best way to tell you something more about this is to write down a bit from the introduction of the book "Yuganta" by Irawati Karve in which are essays on events and characters of the Mahabharata. (Sorry - this next is bit long for a post - but I thought it would be interesting for you - it was for me.)