Often you'll watch a zombie film that's based on a good idea, but then the production values and script just kill it. Stacy seems like the kind of film that should do that, but it somehow manages not to fall flat. Maybe it's the acting, which, although in general not great, is not too subpar, either. Maybe it's the attention to detail and the humorous looks at the the world they create, like the infomercials for the "Bruce Campbell's Right Hand #2," a chainsaw designed especially for the layman who needs to kill a Stacy (zombie schoolgirl). On top of that, the makeup effects are impressive, as well.
But then, the question may be, How can a film with a good idea, not terrible acting, attention to detail, and good makeup only get 2.5 brains? Stacy still has some characteristics of the bottom of the barrel. The film suffers from camcorder syndrome, and sometimes the acting does indeed go right down the tubes. There are also some scenes that are unnecessary, and you find yourself waiting for them to end.
But anyway, what really makes Stacy stand out is the idea. The world is turned on its head when girls between the ages of 15 and 17 suddenly begin dying and then coming back as zombies. The term "Stacy" is coined for these zombies, and it becomes the accepted term. The world tries to adjust as best it can, passing laws dictating what can and can't be done to a Stacy, like who can kill one, and forming a crack Stacy extermination force called the Romero Repeat Kill Swat Team. The repeat kill is a matter of much concern for young girls, since they want be sure that someone will put them out of their misery once they come back.
The film has various subplots, but the main one concerns a puppeteer (Toshinori Omi) and the schoolgirl (Natsuki Kato) who asks him to repeat kill her when it's time. She has several days before she dies and changes, and the story focuses on their relationship as the time of her death nears. The other subplots contribute more standard zombie movie fare.
How impressed you are by the story may depend on how deeply you are willing to read into it. On the surface, it's just about zombie schoolgirls. At the other extreme, it's a commentary on the objectification of young women in society. The director's actual intentions are probably somewhere in between.