The series is set 150 years after humanity lost half of the planet due to a mysterious "black fog." To contain the "black fog," 3,000-meter-tall (about 10,000-feet-tall) walls were erected to split the world, and the half of the human population that survived now live in cities called "arks
There's not really much to say about this anime other than it has a confusing premise for a story despite being an OVA of about 11 minutes long, short character introductions, not so pleasant animations, and very poor directing from Shin Itagaki. To be frank, I don't even know if this anime was even worth making an adaptation, if its only going to be extremely short and only feature one episode. It almost makes me think that the production company had nothing better to do with their spare time other than to make this. It really served more like a promotion rather than feeling like an OVA. Overall, I don't recommend watching this anime unless you absolutely feel like watching it.