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I watched this as a teen, so it is probably dated before 1995. If I remember correctly, one of the initial scenes was set on a space-ship and there were some heavy vault-like doors. I believe some kind of accident causes at least one of the sealed doors to open and an alien breaks free. Somehow, the alien (if my memory serves me right it was just referred to as a "Xenomorph" later) gets to Earth and thats where most part of the movie happened. Most likely it was "present" Earth, no future setting.