You are Parker, a weary miner who works for the corrupt Ultor Corporation. You reside on Mars, sometime in the future, and when a plague spreads through the mining community, some of the Ultor employees uncover a massive conspiracy. Thus ensues a battle by a group of lowly workers, headed up by the educated Parker, to expose the deadly experiments Ultor has unleashed onto its work force.
Plain and simple. It was, and still is, the best damn game in the series. None of the others even come close to matching it.