The world of Rez is set in the near future on a massively overpopulated Earth with society crippled by crime rates, and on the verge of meltdown. In an attempt to resolve the chaos, a new global network was created: Project-K, overseen by a self-aware artificial intelligence named Eden. However, the increasing information flow and infection by a virus (the main character) confused Eden, allowing her to begin questioning the meaning of its existence and the consequences of her actions. Trying to fathom the paradoxes that surrounded her and realizing the power she possessed, she shut herself down.

The player character and protagonist is a hacker virus unofficially named Swayzak, traversing the Project-K network on a mission to re-awaken Eden and to reveal the true being at her core by progressing deeper through the system, and destroying the other viruses, anti-viruses, security programs, defense systems, and firewalls within.