Aziraphale found Crowley sitting atop a chimney, about a hundred metres away from the outskirts of the fire. He was watching the flames swallow the wooden houses before him, unstoppable and relentless. There was a peculiar, conflicted expression on his face. Aziraphale hoped that the demon was having second thoughts about setting fire to London. It would be much easier to get his help if he was. He appeared on the rooftop, beside the demon. Crowley didn't look up.
"Crowley," Aziraphale said softly, reaching the demon.
Crowley didn't answer, he just watched the flames. Was it horror in his snakelike eyes; or was it pleasure? It was so hard to tell through his glasses.
"Why Crowley?" Aziraphale's words of persuasion left him, and he thought only of the millions of people dying down there, in the choking smoke.
Crowley looked up at him, confused. Aziraphale tried again.
"Did you start the fire?"
Finally, the demon spoke, his voice bitter.
"Of course, you would assume that, angel."
"So, you did?"
"No." Crowley stood up on the roof.
Aziraphale thought that it sounded truthful, but he didn't want to trust the demon until he was sure he was innocent. He stood his ground, determined to find the truth.
"Then why were you in Pudding Lane last night? And why are you here now?"
"I was looking for you, you stupid angel!" Crowley's voice was suddenly a shout of despiration, "because you were here, and there was a fire and…and…"
His voice trailed off. What was he supposed to say? That he had worried that Aziraphale would be discorporated. That he would lose his only friend. Because that's what they were, no matter how many times Aziraphale denied it. Friends.
But his words made Aziraphale angry too. Because of what they suggested. Because they couldn't be true.
"Well you didn't have to," Aziraphale shouted.
They had to shout now. The flames were getting closer and they burned away their voices from the air itself.
"I don't need helping by some demon! And I don't need to be lied to! As if you, a demon, would come to help me, an angel. We're hereditary enemies, don't you understand that? You just trying to get me to fall, so you can gloat!"
It was everything Aziraphale feared, pouring out in a helpless rush of frightened words. He immediately regretted it. Crowley took a step back, his face a mask.
"Fine." His voice was hollow, quiet.
He turned away from Aziraphale and went to leap of the roof, to give up on London and drink till he was unconscious, when something stopped him. Something inside his demonic brain wouldn't let him give up on the city, on the angel, no matter how hard he tried. He turned back to Aziraphale, who was standing hopeless, watching him go.
"What can I do to prove it to you?"
Now Aziraphale looked shocked. But then he steeled himself. The fate of London depended on him. Heaven wouldn't like it, but in his heart of hearts, Aziraphale knew it was the right thing.
"We can save London Crowley," he yelled over the flames, which were now licking the very building they were standing on, "if we work together. We can't stop what had happened, we might not even be able to stop the fire spreading, but if we join powers, we can save the people. Only six have died so far. We could save the rest."
He stood, looking up at the demon, ever hopeful, despite everything. Crowley looked down at him. He should refuse. The people should die. But Aziraphale needed him. He couldn't leave his only friend.
"Then let's get on with it!" he shouted, a manic gleam in his eye.
…
It was Sunday the second of September 1666 and an angel and a demon were walking down Pudding Lane. It was unrecognisable. The buildings had become blackened husks and the air was full of thick, dark smoke. But they walked on impervious, unafraid. They were going to save London.
Later that day, a demon would receive a commendation for causing one of the most famous fires in history and an angel would be praised for saving so many lives. Nobody suspected a thing. And an arrangement would become an impossible friendship, that would one day save the whole world.