Story

 Occupants of The Dae-su, an ultra-modern office building downtown, begin exhibiting abnormal, dangerous behavior. Following reports that the phenomenon is spreading through the building – as well as speculation that anything from a terrorist attack to a virus to a disgruntled employee might be responsible – authorities quarantine the building and emergency response protocols are initiated. The player's character becomes trapped in the building along with hundreds of others, many of whom begin to fall prey to the transformations that are ravaging the building's populace, turning them first into crazed psychopaths and then into something much, much worse.

The player's character must, at the very least, learn what's behind the mass mutations in order to avoid becoming part of them, as well as escape death at the hands of the creatures, the authorities, his fellow quarantined, and the collapsing building. Ideally, he'll also save the world by thwarting it's invasion at the hands of creatures from another dimension, not to mention rescue his partner and resolve the conflict he has with his antagonist, both of whom are also quarantined in the building with him.

The game's title, The Epidemic, refers to the invasion into our world by unknown, unseen creatures from another, via the possession and transformation of those humans unlucky enough to become infected. The infections are initiated via the recital of an ancient text. All within earshot for long enough to succumb to its effects (albeit to varying degrees based on the length and volume of their exposure) become the playthings, both behaviorally and biologically, of the presence growing inside them. Initially, the recital and thus the transformations, have a single point of origin, a savant whose superhuman memory is thought to contain the last existing, whole, accurate version of an old book, a book that's purpose is to make our world suitable for habitation by the occupants of the another. 

The savant was brought into the Dae-su building by the cultists who worship the extra-dimensional beings as gods, and have done so for generations, in order that the information that's trapped in his head be recorded. Their goal is that the recitation be broadcast – and the transmutations take place – on a massive scale. The transmutations form the backbone of The Epidemic, in that they are the generative system by which the nature and number of game's enemies, The Horde, are determined. 

Character story lines – and their individual NPCs, both partners and enemies – as well as the creatures' origins, manifestations, and goals are addressed further on. Suffice it to say that by the end of the game the player will have survived successive waves of possession and transmutation; weathered increasingly desperate and dangerous authoritarian responses to their perception of the problem; navigated back and forth across the increasingly structurally unsound building; saved, killed, protected, fled – and agreed to disagree with – the infected and uninfected, stranger, friend and foe; and found themselves triumphant over – or lacking in the face of – the threshold of the apocalypse.