Arya goes to fetch water for Lord Bolton's basin and passes beneath the heads of the former Lannister host.
Gendry bumps into her, and he admits to hating the Bolton bunch more than Ser Amory's men. Even Arya wishes she'd gone with Jaqen H'ghar now.
She brings the water to Bolton's bedchamber while Bolton is receiving his routine leeching—and he's not even consulting a licensed barber.
Arya overhears Ser Aenys discussing Bran and Rickon's deaths.
Bolton decides he will go hunt wolves.
After the lord leaves, Arya tidies his chambers and notices a map. She quickly figures out she is not far from Riverrun, then she's off to train in the godswood.
After Bolton orders his supper, Arya asks if he will take her with him when he leaves Harrenhal. Bolton excuses her insolence for asking a question and answers just this once: She will remain at Harrenhal with Vargo Hoat.
That night, Arya goes to Gendry and tells him about her escape plan—Gendry will steal the weapons, Hot Pie the food, and she'll borrow some horses.
Arya tricks the stable boy into giving her the horses by pretending they are for Lord Bolton, then she meets up with Gendry and Hot Pie; both have completed their tasks.
Arya goes to the gatehouse and pretends to give the guard a silver coin from Lord Bolton. She drops it, though, and when he bends over to pick it up, she slits his throat. Cold blooded.
She whispers, "Valar morghulis" and then signals Gendry and Hot Pie.