Arya, Gendry, and Hot Pie are marched to Harrenhal with the rest of the Mountain's captives.
The Mountain and his men rape, murder, and otherwise ill-treat the captives the whole way, all the while questioning them on the whereabouts of Lord Beric Dondarrion, as well as any caches of food or gold.
During the march, Arya begins creating a list of the people she hates so much she wants them dead—she says their names every night before sleeping like super-scary prayers.
When they make it to Harrenhal, the trio is separated.
Arya gives a false name, calling herself Weasel. She is given a position running chores for Weese, the understeward of the Wailing Tower.
Weese states he can smell the emotions of those in his employ. And all he wants to smell on them is fear.