One of the surgeons Joel killed had a daughter called Abby

The Firefly massacre destroyed her whole life, she's been looking for revenge ever since

Knew who Ellie was because of her dad talking about their shot at a cure, figured out Joel from Firefly records

Cult attacks Ellie's settlement and she, Jesse, and Dinah are part of the group sent to track down and attack it - Jesse is killed during this

Ellie is now violently angry and goes to Seattle looking for the cults home to wipe it out, meets Joel there because he heard about Jesse and came back to help her

Abby finds Ellie because she's been in conflict with the cult too, so when Ellie hunts them to Seattle and is set to attack them with Joel her group find them and attack

She kills Joel herself, takes Ellie back to camp debating what to do with her since for all her hate she doesn't blame her for the massacre

Ellie butchers her group to escape and Abby is now motivated to hunt her down

Second half of the game is spent as Abby chasing Ellie

Ellie continues hunting the cult while you chase her down

Ending is Ellie and Abby fighting to the death

You kill Ellie as Abby, and she leaves while realizing she's not really "won" in any meaningful way since all she's done is get more of her loved ones killed for the sake of her killing Joel and then Ellie

Ending is "cycle of revenge" with the loose implication Ellie's friends will hunt Abby now in return