For a one shot session with Merc:2000 characters, you could use one of the adventures in "Twilight Nightmares". I find these adventures difficult to fit in any Twilight 2000 / Merc 2000 campaign, but some of them are good enough for a session. I remember one in particular, "This Ain't No Weather Balloon", about the rescue of an important businessman in South America, with UFO included. Not bad.
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