The zombie apocalypse is a funny theme, as so many people attempt to tell the story their way via video game. There's the cartoon, run at your house, and you flick them away; there's the zombies-breaking-through-your-house-and-you-have-shoot-them-down zombie apocalypse story, along with many more that are too long to cover.
So what's BioDefense all about? Well, for one, it's all about survival (this seems to be a recurring theme). But in this case, they're trying to attack your energy plant, and you must set up towers and energy towers to defend your plant as long as possible. It's okay though, you don't need to panic; there are machine guns and rocket launchers available at your fingertips, so fighting off these zombies would be easy, right?