Joe Dante directed Homecoming last season, and this one is a huge step up from that point. Of course, while I enjoyed Homecoming, I recognized that it wasn't especially scary. The Screwfly Solution plays like a Stephen King story, but with a little bit more sci-fi than he usually indulges in.
Around the world, males start killing females. It is unexplained, although the perpetrators claim God told them to do it. A scientist (played by Jason Priestly) attempts to figure out what to do about this, while his wife heads to Canada in an attempt to escape the whole mess.
The episode is strangely unbalanced. With the plot supplied, it could easily fill a feature film (or miniseries), however, the second half of it feels very slow. This is a side effect of having to trim events down to only the highlights of her life on the run.
There is considerable horror in the story, but the sense of horror decreases as the story continues through the second half, and we are filled with a sense of resignation to the downer ending that is impending. I should clarify though – the first half of the film is excellent in pacing and horror.
An earlier reviewer wrote "why did they go about it in such a gratuitously sadistic and misogynistic fashion?" This question is clearly answered by the movie. The easiest way to eliminate a population is to prevent it from breeding. If males were wiped out, it'd still be possible to continue the species. Without females, the species has no chance of survival. The gratuitously sadistic aspect of their method of infestation removal is strictly in the eye of the beholder.
Update: It's actually even better than I thought. I returned to this episode, and on a repeat viewing, knowing the ending, as well as having grown up a little more since my first viewing - this is really, really good. There are a variety of clues dropped, and the pacing isn't as much of a problem as I thought it was. The second half changes from outright violence to a sense of tension, and more horrors are implied.