Takes place between episodes 2.14 (City of Walls and Secrets) and 2.18 (The Earth King).
In Ba Sing Se, nobody stared. His scar was nothing. Boys younger than he came back from the warfront all the time with worse. The walk from his apartment to the teashop where he waited tables was thick with the blinded, the mute, the crippled.
Zuko had never seen battle. He had watched his father's advisers push marker-pieces across maps of the lands encircled by the uncrossable vastness of the ocean, heard them speak blandly of the casualty reports and structural damage. He had dueled, yes, fought a few skirmishes against the Avatar and his companions, yes, even been a bit player on the assault against the North. But he had never been in the thick of it, not the kind that decided the fate of cities and nations.
Once, foolishly, as a little boy, he had asked his uncle what it was like, expecting a long, drawn-out tale of heroic daring guaranteed to keep him up past his bedtime. But instead his uncle had gone very grave, suddenly looking so pinched and old in the flicker of the firelight. "If you never see it, my nephew, I will die a happy man," he said, and left without another word, forgetting their little ritual of extinguishing the lamps by turns until the room was left in velvety darkness.
It had hopelessly perplexed his child's mind, raised as it was on the steady beat of fanatical worship of the tank and the fist that made every timber in the palace hum.
There was a refugee sitting in the muck on the street outside the door to the teashop. A mother, he saw, clutching her ragged bundle of a child to the rough and ravaged flesh visible beneath the collar of her robe. He would not have stopped, but she glanced up at the sound of his tread on the cobbles, and their eyes met. The look she gave him was neither pitiful nor pleading. It was blank. Dead. Deviod of hope that she or the too-thin baby she held would ever wake from the nightmare of insatiable flame that swept her world away.
He fished a copper piece out of his pocket to drop in her begging bowl, and almost, almost understood.