The distant future, 5012. 
The sudden aerial invasion of Earth by ＜Aliens＞ and
their creations ＜Machine Lifeforms＞led mankind to the brink of extinction.
The surviving number of humans who took refuge on the moon to organize a counterattack using
＜android＞ soldiers to recapture Earth. 

However, the war reaches a stalemate as the ＜Machine Lifeforms＞ continue to multiply infinitely.
In turn, humanity deploys anew unit of android soldiers as an ultimate weapon: ＜YoRHa＞ 
Newly dispatched to Earth ＜2B＞ joins＜9S＞, the analyst currently stationed there, where amid their mission, they encounter a myriad of mysterious phenomena... 

This is the story of these lifeless ＜androids＞ and their endless fight for the sake of mankind.
 
(Source: NieR:Automata Ver1.1a Official USA Website) 
Note: Episodes 9, 10, 11, 12 aired back-to-back on July 23, 2023.