In the early parts of the main story of Dying Light 2 Stay Human, you have two choices. You either need to activate the water tower for the Peacekeepers and complete the storyline in Old Villedor with them or transfer the power for the Survivors.
If you choose the former option, you’ll find yourself having to do the Into the Dark quest. By the end of it, Anderson, one of the Peacekeepers who aided you before, will need your help. You can thank her for the previous time she lent you a hand and leave her to her own demise or choose to help her.
Unfortunately, this choice doesn’t matter all that much, except for the sake of you and Aiden’s conscience. This is because no matter what you pick, Anderson will end up dying anyway.
If you decide to continue on your own adventure and ignore her, you will simply proceed to the next task in your main story.
But once you decide to help Anderson, you will be tasked with heading out to one of the military zones to activate the generators. One of them is cordoned off by a wire, which will require you to do some parkour. The second one, though, has a herd of infected guarding it and you’ll need to slaughter them to proceed.
When you turn both generators on, the elevator power will return. Call it and you’ll find Anderson. Unfortunately, she will have already turned into a zombie and there’s nothing that can change this. You can put her out of her misery and take the loot as a consolation prize.