The premise behind the story is that machines have become self-aware and taken over the world. Humans are tolerated in a sort of preserve zone. There's an underground resistance group called Eleven, with which our heroine Karen is associated. She's constantly running from machine enforcers doing their best to kill or capture her.
Sounds like a pretty straightforward fear-the-robots science-fiction dystopia, right? Well, not exactly. The machines act more like conquering humans than robots. They act like gangsters, buy human food, listen to singers - you could replace them with conquering aliens or Nazis or whatever, and not miss a beat.
Then there's the mystical aspects. Karen needs gun repair at one point, and it's handled by an old guy in a backstreet shop. Pretty normal, until it turns out that he's really some kind of priest who takes the gun to a big temple and performs some kind of ritual on it. Then there's Karen's dead sister, who takes a much more active role at times than the average deceased sibling. And the Eleven member who grows replacement bodies in her refrigerator and somehow transfers her mind to them upon death. Not what you'd expect from a machine-ruled future at all.
Since it's so short, Karen Senki doesn't have time to fill in a lot of details. Most creators would tell a simple story in the limited time, but not here. A lot of stuff is simply left out, assuming the viewer will connect the dots. No character-developing side scenes, or background explanations. Drop a hint or two and move on, no time for details. The back-story is filled in a bit in later episodes, but again it feels like a few key scenes rather than a full story.
There's a big final battle, of course, saving the world...or at least the part of it where the humans live. No real attempt is made to resolve any of the big questions, though, or go after the machine mastermind. Which is not surprising, in a short computer-animation showcase, but still a little disappointing.
Karen Senki is pretty to watch (as long as you're OK with CG animation), and reading between the lines of the story is an interesting exercise. It doesn't resolve many of the questions it raises, though, and the ending isn't particularly satisfying. Not a bad way to kill an extra couple of hours, but not worth going out of your way to see.
