The whole film is one giant coversation between two couples. The son of Nancy and Alan (Kate Winslet and Christoph Waltz) hit the son of Penelope and Michael (Jodie Foster and John C. Reilly) with a stick and the two couples are trying to work out a resolution.
At first the two couples seem quite different, with Penelope and Michael coming off as easy going liberals, while Nancy and Alan are uptight and busy conservatives (with Alan always being preoccupied with phone calls from work). However, through the course of the conversation, as tempers begin to flare (and Scotch is introduced), the four of them gradually reveal their true selves and we find out they aren't much different at all.
I thought that Carnage was a real blast. It was fun watching the polite facade slowly fade away over the course of the film until there was nothing left but total anarchy.