The mission files are:

- <mission name>.level
- <mission name>_scripts.engscr
- <mission name>_zones.engcfg
- <mission name>_loc_data.text

The ".level" file has links to the other files, so you'll have to edit the text in Notepad or something,

"It's fascinating to think that all around us there's an invisible world we can't even see.
I'm speaking, of course, of the World of the Invisible Scary Skeletons."