This episode is part of the Avatar Book Two - Earth
Avatar The Last Airbender Episode Guide ~ via Wiki
Aang and the crew are shocked to find Omashu captured by the Fire Nation. They sneak in and are caught by Fire Nation soldiers, but released when pox marks on Sokka create the illusion of an illness. They then meet the resistance and allow the whole city to escape by creating a fake epidemic with the same pox marks. While they are leaving, the Governor of Omashu’s son, TomTom, accidentally leaves with the Earthbenders who are willing to trade him for King Bumi.