It could be that the short story is set in a small town. There is a problem, like a drought or a feud between families. Then a stranger arrives. This stranger doesn't seem to be anything special at first. But as time passes, they start to bring the community together. They might share resources during the drought or help resolve the feud through their wisdom and understanding. This stranger is the 'angel in disguise' in the main plot of the story, and the story is about how they transform the town.
I'm not sure specifically as there are many possible plots for such a story. But it might revolve around a character who is initially unrecognized as a positive force (the angel in disguise) and then gradually reveals their true nature through their actions.