Ooo boy...
To start things off, Doom is one of the first games that gave birth to the first-person shooter genre. In this game, the moon bases that were set up on the two moons on Mars, Phobos & Deimos, came under attack by the forces of hell. Eventually, the Deimos base literally disappeared without a trace, & soon, a squad of space marines get sortied out to deal with this issue. Bad thing is, hell's demons are a force to be reckoned with, thus all but one marine in that squad have gotten wiped out. In Doom, you play as that lone marine, & have to fight through hell's army of possessed humans, imps, & assortment of demons to figure out what has happened to Deimos & eventually, put an end to this invasion.
It doesn't surprise me that you've never heard of this game - it's older than you are!
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