It's all about... energy drinks
Here's the premise. In the fictional Sunset City, energy drink mega-corporation--Fizzco--has created its latest concoction: Overcharge Delirium XT. It delivers samples to almost everyone in the city, and... pretty soon the whole place is overrun by bizarre monsters called 'the OD'd'. It's probably not a coincidence. You play as a humble cleaner, who is busy doing his job when the disaster happens.
Fizzco is keen to cover up the disaster, so the company (conveniently) cordons off the city. That leaves you tapped with a bunch of energy-drink mutants, leaving you no choice but to... have fun. The devs are keen to stress that this game is all about living out all your wildest fantasies inside a chaotic city. It isn't about survival, ammo counting, or saving the world: it's about blowing stuff up and being a video game badass.