Nier: Automata opens with 2B and 9S opening the way for a future incursion into machine territory, working to clear out machine threats for the local Resistance. With help from Anemone and later Pascal, 2B and 9S defeat multiple machine threats. During their missions, they witness the birth of Adam and Eve, physical manifestations of the machine network who reveal that their alien creators were destroyed centuries ago. 2B and 9S also encounter A2, a wanted YoRHa unit. It is eventually revealed that the machines are beginning to feel emotions and gather in groups, and some like Pascal's group have grown tired of fighting. 9S is eventually captured by Adam, who 2B fights and kills. Deprived of his brother, Eve goes mad with grief and drives the machines under his command into a frenzy. 2B and 9S kill Eve, but 9S is infected with the machine's logic virus, forcing 2B to kill him. Despite his android body's death, 9S's personality is saved in the local machine network, allowing his transfer into a temporary machine body. Ending A is narrated by 2B and shows 2B and 9S both surviving.
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https://www.youtube.com/watch?v=rgbCwUs5lV4