The two movies therefore, unsurprisingly, have a lot in common, including brilliantly, carefully plotted, complex stories, minimal exposition in dialogue, and a low-key ambience that goes far below and deeper than just low-budget. But the two are also very different - not only because Primer is about time-travel, and Upstream Color is about, well, I'll get to that in a moment, but because Upstream Color has a much richer emotional current.
And, actually, that current gets to what Upstream Color is and is about. It's a relative rarity in science fiction movies - unlike time travel - a highly literate, philosophical kind of biological science fiction. Biology in science fiction is common - we encounter it every time we see a movie about a mutation gone amok, and often in narratives about aliens. In many cases, such movies are pulp-horror or superhero stories.
There is a horror in Upstream Color, but it's more quiet, under the surface, and therefore disconcerting if not viscerally frightening, though Upstream Color has some of that, too. The story, in a nutshell, is a about a worm-like organism, whose life cycle entails orchids, pigs, and human beings. As in all parasitic life cycles - a real part of our natural world, not just science fiction - the hosts do their thing, live their lives, with little or no awareness that they're doing the parasite's biological bidding. Upstream Color explores this, harrowingly, subtly, compellingly, for humans, including our two central characters (with good work by Amy Seimetz as Carruth's character's partner), who discover a powerful attraction to one another, and various people ranging from a swindler who's also a kidnapper to pig farmers and gardeners.
The result is an indelible Blue Velvet-like tableau of life under the surface, which feels incomplete, just as did Primer. But that's part of the power and charm of these movies.