A bunch of friends, most of whom sort of dislike each other, try to have a nice and relatively sane dinner party the same night a comet is expected to fly overhead. Unfortunately, the comet screws everything up. Because comets. The lights, phones, and Internet all suddenly stop working. Down the street they can see a single house which seems to still have electricity. Upon more investigation, they discover that it's actually their house, occupied by alternate versions of themselves.
That's the basic premise behind Coherence. From the moment our gang of pals discovers their situation onward, the film acts as a kind of mumblecore version of a Futurama episode. If you have a hard time figuring out whether that sounds awesome or awful, welcome to Coherence's strangely appropriate dual nature as both a great and sometimes not so great film.
To keep with the television metaphor going, Coherence is a bottle episode movie. We have a small cast and one location and that's about it. When we first meet them, they lack a certain definition other than the initially distracting fact that one of them is played by Nicholas Brendon from Buffy the Vampire Slayer. With time, however, some of them start to stand out. Only some of them, though. There's a Kelly McGillis-like older lady and her college professor looking husband, both of whom annoy but in a highly realistic fashion. There's also a lady everyone throws shade at because she used to be with another friends' current boyfriend or something like that (this relationship drama stuff much easier to see than to discuss). At first you feel bad for her, but little by little she reveals herself as totally shade-worthy. I'm already blanking on pretty much everyone else in the film. It would have been nice if these individuals were more developed and memorable, but it's doesn't actually hurt the film that much because their predicament is interesting enough to take this particular weight off the movie's shoulders.
As we watch this group try to grapple with the gigantic strangeness down the street, we come across the same sort of paranoia we might expect from such a setup. Tensions flare not just from activity involving the other house, but from drama solely within the main house as well. Once it becomes apparent that some of the friends could actually be sneaky replacements from down the street, we have to sit through a lot of yelling drama. But even with those concessions, things never resort to the kind of chaos you'd expect. One of my favorite things about the film is how logical and even-handed the group is as a whole. They still screw up and do dumb things, but they never devolve into a mob.
Except for Nicholas Brendon's character, that is. Upon figuring out what's really going on, this guy turns into a violence-hungry madman with a speed that could only be described as snicker-worthy. But then you realize his concern stems from the realization that there's another him out there, which means there's another especially violent hungry madman out there. Stuff like that happens all the time and constantly keeps Coherence ahead of your expectations. Whenever you think it really screwed up, it kind of saves itself.
In the end, Coherence doesn't add up to much more than a fun Twilight Zone episode, but that doesn't mean it's bad or not worth your time. As a talky science fiction film based on quantum mechanics, one that frequently comes up with good enough ideas to battle its occasional faults, Coherence is the kind of film you really want to root for.