This episode is part of the Avatar Book Two - Earth
Avatar The Last Airbender Episode Guide ~ via Wiki
Aang, Katara, Sokka and Toph are chased by an unknown machine, which prevents them from stopping to sleep. After Toph and Katara keep arguing, Toph leaves and runs into Iroh, who is tracking down Zuko. After cleaning Appa so his fur doesn’t leave a trail, Aang goes one way as a decoy and Sokka and Katara go another direction on Appa.