It's not a TV series compilation, but a sequel to Season 1, based on vols.4-5 of the manga. It's not as strong as the 2nd movie, but few things are. It's freaky, upsetting, desperately gross and all those other features of Made in Abyss.
It's also a very simple story. (It could never have been a complete season. Four or five TV episodes, maybe.) Our heroes meet Bondrewd, who's basically Doctor Josef Mengele. Seriously, look him up. I think the character might be directly based on him. Here's a quote about Mengele from a former Auschwitz inmate and a doctor:
"He was capable of being so kind to the children, to have them become fond of him, to bring them sugar, to think of small details in their daily lives, and to do things we would genuinely admire... And then, next to that... the crematoria smoke, and these children, tomorrow or in a half-hour, he is going to send them there. Well, that is where the anomaly lay."
Bondrewd did Mengele-level things to Nanachi and Mitty. We learned this in Season 1. However he's also a polite, courteous gentleman who believes in love and has a daughter. Her name's Prushka and she thinks he's the greatest. She's as nice as him, except that with her it's not a mask. Personally, I'd argue that Bondrewd is like our heroine, Riko. They're both obsessed by the Abyss and they both have an impossible optimism. Even after seeing, doing and eating things that have been hard to watch, Riko thinks the bottom of the Abyss must be a wonderful place.
It's still very Made in Abyss. It's surreally beautiful when it wants to be, although there's less of this than in Season 1 because Bondrewd's subterranean tunnels are drab and utilitarian. It has adorable child heroes and cute character designs, without which the show wouldn't have a tenth of its power. It has some Really Bad Decisions. (No, Riko. Not the stairs. No, no, no, no, no.) It has disgusting food. (The stuff that tastes like a wall at least looks normal.)
There's not a huge amount in this film to discuss, really. The storyline's extremely straightforward, although the themes are powerful. (Bondrewd believes in family and regards Nanachi as just as much his child as Prushka. He makes a good point when he says that not being related by blood shouldn't be an obstacle. The cruel thing is that Nanachi still regards Bondrewd as a father figure and they have a number of calm, rational conversations. There's warmth of a sort between them, even after everything Bondrewd did.) I'd thoroughly recommend this film, obviously, although it's not as annihilating as the second one and there's a certain amount of repetition through its last hour. It can be shatteringly horrible, but also charming and lovable.
This series is probably the strongest anime of the past decade. Not everyone will be capable of watching it.