You could describe Kentucky Route Zero in a whole bunch of ways. You could say that it's an adventure game or an art-game, for example. You could say that it's an essentially an interactive music video.
All of these descriptions would be true, but none of them would be accurate.
Instead, the best way to describe Cardboard Computer's opening act of this five-part title would be to say that it's a piece of interactive fiction - though on a scale more like a Choose Your Own Adventure novel than something like A Mind Forever Voyaging. In fact, that's what you're doing for most of the game; choosing your adventure.
You see, while Kentucky Route Zero may be presented similarly to an adventure game, it ultimately bears little resemblance to the rest of the genre once you push past the interface. There are no puzzles to tax your dim wits (not beyond finding locations anyway) and there's no inventory or parser to deal with either.
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Instead, there's just a story being told and a few tools which allow you to bend it to your will. Mostly this is done through dialogue trees and the order in which you visit locations.
This might sound dull, but it's actually no bad thing at all. Storytelling is what Kentucky Route Zero does best of all and by focusing on that to the exclusion of predictable conventions means it's able to shine in a way that it otherwise wouldn't.
It's worth noting that 'storytelling' doesn't immediately denote 'cutscenes' in this case either; Kentucky Route Zero works more than through just simple exposition. It's the culmination of the music, art style, writing and even interface which work to convey Kentucky's narrative. Simple things, like minimalist maps showing only white roads on black backgrounds help communicate how Cardboard Computer want you to see the world; as a journey and not just a series of destinations.
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The tool Cardboard Computer wields best of all though is a sense of unreality and mystery. There's a constant but gentle confusion saturated in to Kentucky Route Zero which makes it both compelling and mystical. Again, it's the little things that do it - poems used for computer passwords and spontaneous changes in which character you're playing.
Boiled down to the absolute basic near-certainties, Kentucky Route Zero is the tale of a delivery driver trying to complete his last shipment. All he has to do is find the house and drop off his boxes of antiques, but to do that he's told to find 'Route Zero' - and that path leads him off on magical tangents.
That's magical in more than one sense, by the way. It's not ever explicitly stated, but the bluegrass soundtrack combined with the art style would seem to place Kentucky Route Zero in America's Great Depression. Couple that with the focus on mystical backwaters and Kentucky Route Zero exemplifies the sort of bleak, magical setting seen in shows such as Carnivale.