Robots it is!
Also we don't necessarily need a story for ipe's idea, we basically just give a small reason as to why the player is here doing what he is doing in the first place, and leave out any sort of story, and also if we go the 'infinite' waves/levels thing, then that is even easier since we wont really need an ending.
What about if we have the infinite thing, and make the whole game a "you are going to die, this is your story/ this is how you died" thing. We could have the town being over run as ipe suggested, and basically you would have to defend the town, to get back to the main game or whatever.
Depending on how well you defend the town, would decide how much 'Health' the town has left. We could possibly use this as a 'lives' system. Town blows up, game is over.