display only one random post excerpt

I’m trying to apply the recent-posts-with-excerpts plugin inside the Vulcan theme. Where the widget is placed I only have room for one post excerpt. I’d like the widget to display the excerpt from a random post within the selected category each time a page loads.

Can you guide me as to where and how to make this change?

http://wordpress.org/plugins/recent-posts-with-excerpts/