Doom is one of those rare naratives that works both as a violent video game and a violent movie. Here, in the film version, you can see why. A weird virus/mutation/science whatsit has gotten loose in a research station on Mars. It's turning everybody into angry monsters who shoot tongues. But that's not all: apparently the shooting tongues have the ability to tell if you are evil or good based on your genetics, and will target you accordingly. Here you can see the full bizarre wonderfulness of the tongue-shooter, who is chasing after one of the researchers and a military guy, but gets stuck in a forcefield.
@Ghede: I know this makes me a bad person who should be spanked with alien death tongues, but I actually LIKED the genetically whacked monsters better than demons. They were just so hilarious with their "I can tell whether you are good or bad by sniffing your genetic material" thing.
Also, I would note that I have the UNRATED version of Doom and it includes a lot more freaky naked chick gore, which I wasn't expecting at all.