search one category only

Does anybody know of a plugin that can be used to search within one category only? I’d prefer a plugin rather than messing with theme files.

I want to upload abstracts (summaries) of college dissertations on my site. I’ll probably do this pasting them into individual posts with a common category of ‘dissertations’.
So when someone goes to a particular page of my site, I would like for them to be able to search within the ‘dissertations’ category only.

I’d be grateful for any suggestions. Thanks