It started really well and had a great minimalistic and claustrophobic setting on a great set. Which I must praise highly in every part of its design!
There was so much tension in the beginning bordering on horror. I could not stop watching as long as there was still this mystery hanging over everything. You didn't know how this was gonna end, how it was gonna be explained. Would everything have a plausible scientific explanation or was there something supernatural?
In the beginning it was such clever writing. Every element was there for a reason. Both an in-story reason as well as a storytelling one. Unfortunately in the end there were a few illogical decisions (like a characters motivation to give blood just to kill the saved person moments later - although I understand this might have been necessary to explain why Ryu took so long until vomiting water) and too many elements were revealed too early, so the tension kind of rippled away.
Despite being already rather short they could have shaved off at least an episode worth of time from the beginning of the second half.
And the very ending was just too much. Walking on the moon completely unprotected is not only ridiculous but does not even fit this series very own logic.