Here's the setup: in a film that's more or less 'E.T. by way of Cloverfield' (and that's not necessarily a bad thing), there's a bunch of kids in the 70s making their own zombie movie, and after they accidentally witness a military train derailment, all sorts of suspcious things start happening, and before long there's a whole military coverup (it seems that with the exception of war movies, and even then not always, depictions of the army in movies are pretty much universally negative). It starts off with a great premise, likable characters, a well-executed 'Romeo and Juliet' subplot between the kids, and an intriuging, well-built-up mystery... and then, two-thirds of the way through, it shoots itself in the foot in spectacular fashion.
Not only is the conspiracy explained, it sets in motion a string of sappy, trite, cliches leading to one of the biggest 'you've got to be kidding me' endings I've seen in a long time, and you'll see all of it coming a mile away. That said, the first part of the movie alone is well worth the price of admission, and my advice is to walk out of the theater when the aforementioned reveal scene is over, and if you're really interested in finding out the specifics of what happens in the disappointing ending, there's always Wikipedia.