It is set after the movie Parasite Eve, and makes vague references to it. The story is based on the notion that mitochondria have DNA and therefore can evolve. From the day eukaryotes have incorporated mitochondria, they have been evolving, and now mitochondria is taking over from within. One character in particular, Melissa Peace, is completely taken over by Mitochondria, mutates into some thing who calls herself Eve, and begins wreaking havoc on New York. Using her Mitochondria powers, she can burn people, turn them into slime, or mutate animals to do her bidding. Only Aya Brea is immune to the effects of Mitochondria, and it is up to her to save humanity.
ParasiteEve has a guns system where you can use items to swap around properties of different guns and make yours stronger. Shotgun rifle? No problem once you find the gun with the "shotgun" attribute! Shotgun rifle with acidic, tranquilizer bullets? Sure thing! All of this, of course, is CompletelyRealistic?.