If you play it right you should be able to finish a whole campaign in less than an hour, under two on expert.
Each campaign is its own story, loosely tied together with fan speculation and a few of the lines the characters say. They are meant to be stand alone "movies" in a quadrilogy. I'd say the story is fleshed out about as well as most Hollywood zombie movies (which is usually little to none).
Right now I'm playing Hammerfight, which is on sale for $5 on Steam right now. I played the beta when it was still in development and then it fell off my radar for a few years. There is something refreshingly SNES about it, but the physics of the game will make your mouse feel heavy, which is great. There is a bit of a learning curve. You will likely fail levels two or three or four or five times before finally beating it and moving on. The story can be a bit vague at times, but the gameplay is really fun, so I like it. It's a definite "go buy it now" game at $5.