For the most part it does a decent job walking the line. Though, at times it almost feels like two separate movies. You'll have a classic horror scene, and then switch characters and feelings to the science fiction aspect to explain what just happened. About halfway through the movie, things start to fall a bit apart. The horror element mostly disappears, and the movie losses steam. As is too often the case with science fiction stories, it ends up feeling like the story writer didn't know where the story was going to take us, and we end up muddling through to the end.