A beautiful stop motion movie, all the more impressive that it's been mostly done by one guy who first did the short version all by himself.

Background story: humans are now immortal, but can't have children. They used to have some genetically modified clones to do all labour, but they rebelled and retreated below the surface. Hundreds of year later humans send an explorer there to see if they can find some info to be able to procreate again.
But that's just setting the world above the surface, that we don't see after the first minute.

The actual story is the explorer being hurt constantly while travelling, losing his memories in the process, then being put in different mechanical bodies as the locals fix him.

There's a neat original visual style to the underground world, a whole ecosystem of creatures living there, and we see him interact with different part of societies, getting some hints on how this underground world works.

The only thing a bit annoying is that they all speak a weird guttural language that is not really pleasing to hear. Guessing it's some kind of modified Japanese, missing syllables or sounds, maybe reversed ? And even if it is not really pleasing it is actually a great idea.

The adventure part, let's call it that, as some of it includes going to buy mushrooms (kinda mushrooms, that grow on disembodied torsos), is pretty varied and nicely paced so that you never lose interest.

And there's still time to develop some side stories, have interesting side characters, some action and a bit of humour. There are a lot of good ideas and creativity put into this.