You're a spartan stationed at Omega Outpost, a satellite space station between Earth and the Moon. Omega Outpost has suffered a serious hit from the main Covenant Cruiser, and now it's up to you (the only survivor) to thwart the incoming wave of covenant interceptors and retrieve the top secret X-2 fighter from the cruiser.
Good luck!
(This is the beta version, whose scnr needs alot more work. Unfortunately, I can't expand the map with Eschaton anymore, because it throws an error at around 61% into the expansion. Also, SwordEdit breaks the map, so I can't use that as far as I know. I'm left to using what scen and mach tags I can scrap together and working with Eschaton's scnr editor, which is time-consuming at best. Another version will be released with the much-improved scnr.)
I'm waiting for the upload to finish at HDM, then it'll be accessible at the Mod Database.
EDIT:
Download beta version
Either you are groping for answers, or you are asking God and listening to Jesus.