Branch A is the first ending in the game NieR: Automata. The game is played through 2B's perspective. This ending is mandatory for continuing the rest of the game and unlocking Branch B.
After you have defeated the boss(So-Shi) in the Abandoned Factory, you will have to report back to the Resistance Camp. Once you arrive, the area will be on fire and destroyed while Resistance Camp members are being attacked by machines. Defeating the machines will then prompt you to exit the Resistance Camp where you will fight a boss similar to So-Shi, which is named Boku-Shi. Managing to defeat it will start another fight, but this time against a machine core surrounded by machine parts.
Defeating it will spawn another near the entrance to Pascal's Village after killing the machine lifeforms attacking. This will then result in yet another boss fight being triggered.
Once you have arrived, you will battle Eve in a series of three phases.
- In his first phase, combat will proceed as normal. Occasionally, he will rise with concrete platforms and throw debris at you. You can reach him by jumping onto those platforms or wait for him to come back down. Once you've exhausted all of his health, he will regenerate it back to full again. After that, 9S will hack into Eve to sever his connection to the network, preventing him from doing it again.
- Phase two starts after 9S strikes Eve, letting you slice off his arm. During this phase, Eve will be airborne and blasting energy spheres at you. This will leave your main source of damage to be from your Pod. Occasionally he will be low enough to attack with your melee weapon.
- In the final phase, you will lose your ability to use your melee weapon and be slowed as if you are in the injured state. He will do the same thing as the second phase, but with hindered mobility it is more difficult to dodge the energy spheres. After exhausting all of his health, the ending cut scenes will play.
After Eve is killed, 9S will be infected by the machine network and slowly be taken over. 2B then will have to kill 9S to stop it from getting worse, and she does this while crying. Machine corpses will then resonate and a Goliath Machine lifeform will rise. It would turn out that 9S transferred his consciousness to that Goliath, which will let him retain his memories. The final scene is 9S holding 2B in one hand similar to how Shakespeare holds a skull while saying "To Be, Or Not To Be." An obvious pun on 2B's name.