This show is amazing, but it can't be discussed without spoilers. I'll start with a quick spoiler-free outline, then get into the dirt after lots of spoiler space. One-line summary begins here.
Natsume is a tough girl who's lost 3% of her body and 100% of her parents. Her dad died in ep.1 and she lost a hand. She's not taking it lying down, though. She wants to be a soldier who fights and changes the world. Unfortunately, she's been designated a bug and denied any work but the nastiest and scummiest, even though this is a post-apocalyptic world of actual giant bugs that regularly attack the remnants of the human race. The last survivors (split into Tankers and Gears) all live in Deca-Dence, a giant robot that stomps the wasteland and is powered by insect blood.
Kaburagi is a tough, gloomy old bastard who thinks nothing will ever change. He dislikes bug-hunting and he doesn't want to help Natsume achieve her goals.
The series's first half is a bit of a slow burn. Stuff isn't changing very fast. Kaburagi takes an interest in Natsume and starts training her, but this is a world where fate hates you. The show's second half, though, is rocket fuel. Natsume's changed Kaburagi. She was right and he was wrong. People make big decisions and change things in a big way, which is awesome as hell.
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Holy shit.
Ep.1 sets up things pretty clearly. The first thing ep.2 does is destroy everything you thought you knew. Mankind trashed the Earth years ago and the rights to the human race have since been bought by cyborgs that don't understand death. Everything we saw in ep.1 is a game. The cyborgs participate in the blood and carnage for fun. It's just a big video game with avatars and non-fatal death (if you're a cyborg), while the killer bug monsters (Gadoll) are being systematically bred and released by the game moderators.
In other words, it's another game anime. There are lots of those, including thoughtful ones like Re: Zero. What's different here, though, is that its virtual reality isn't actually virtual. It's real. Actual bugs are killing actual people, in accordance with the game design. The verisimilitude makes the game more exciting, while in addition it has the side benefit of culling the humans and stopping them from getting too numerous. The cyborgs' avatars are flesh sacks ("Gears") that they pilot remotely in order to mingle with the ordinary humans ("Tankers") and fight Gadoll.
In other words, the human Tankers are just NPCs in someone else's game. They also all have chips to stop them from doing anything the game moderators don't want. Any violator is a bug, to be eliminated as if you'd found an error in a computer program. That's why Kaburagi knows that the world will never change. If anyone ever killed all the Gadoll, the game police would just whip up some more.
This comes with a whole lot of weirdness. The game world is realistically animated, while the "real world" cyborgs look silly and cute, like a mid-morning anime for pre-schoolers. You'll still see them getting blind drunk on homebrew made from monster poo, though, before puking their guts out. (The game police's design is freaky, though.) The show's first half is mildly disorientating and is basically build-up for the second half. There's a literal shit factory. Heroes might say they have noble-sounding motivations, but will eventually admit that these are a cover for their more self-absorbed ones underneath. There are victims who don't want to change the status quo, either because they've realigned their ambitions to fit into it or because they think it's too dangerous to stick their head above the parapet. The current rate of death is acceptable. You can't do anything. Go with the flow.
The grey moral areas make this particularly interesting. The cyborgs aren't evil. They're sort of child-like. It's mankind who trashed the Earth's environment and then sold away the rights to the human race. That doesn't make the current system good, though.
Natsume changes Kaburagi. He learns how to live and challenge limits.
I love this show. It's weird and magnificent and totally anime. It's absurd surrealism, yet also as political as hell. No one else will ever do anything like it. Its last episode will make you cheer. Wow.