Remember Girls' Last Tour? Well, turn that up to 11, and you get this.
The Abyss is a pit, nay a chasm, nay an unholy abomination on the Orth, in which monsters and horrors roam free and tend to eat anyone who wanders into it. Which is what might have happened to Riko's mother, one of the fabled White Whistles who have climbed down through all of the abyss' levels and returned to tell the tale. Until she didn't.
So Riko lives in the orphanage, with all the other kids who have been made parentless by the Abyss. As one of the baby red whistles, she explores the first layer down (of seven), looking for relics to sell on, keeping her fed and watered. Yep, child labour.
Until one day, she finds the greatest relic of all, Reg, a humanoid robot, with extendable arms and a death beam. Also a penis. It's remarkably plot relevant. (There is entertaining child romance. They are bad at it.)
And so Riko and Reg dive into the Abyss, braving death, loss of limb and the occasional slight bruise.
Don't be fooled by the initial cuteness and rather gorgeous backgrounds. This gets dark very quickly once they hit the second layer. We've got hideous beasts that can murder you in twelve diffferent ways, people who aren't entirely human any more, and the Abyss itself is trying to kill you. Ascending through the Abyss can cause nausea, bleeding, loss of your humanity and possibly death.
Our poor cute little deuteragonists are put through some impressively horrible trials, and since they're about eleven (at least mentally in the case of Reg), this is not a nice thing to have happen to them.
Though at least they have nice food.
Good lord they're compelling characters though. I'm more than a little invested in seeing them succeed, and I'm curious enough about all of the questions that the series has raised that I'm considering looking for the manga. (Luckily, there is apparently a second series already greenlit.)
Honestly, one of the best things I've watched in a while, even if the first couple of episodes give it a bit of a slow start. If you found yourself not too enthralled and stopped, give it a little longer.