Media where a random guy finds a problem, solves it, refuses to elaborate, then leaves.

One of those old storytelling tropes is the mysterious stranger or traveller who stumbles into a troubled situation, and turns into an unlikely or prophecised hero. The classic versions are stories of a Knight showing up to defend a village from a dragon that terrorizes it.

One of my favourite example is from the classic vampire book Carmilla. There was a big vampire infestation at a foresty area in Europe. One day a badass knight is passing through that area and hears about the vampire problem, so he finds the source of the vampires, the graveyard of an old abandoned castle, then proceeds to kill all the vampires in their graves while they slept. All except one whose grave was empty.

So the knight climbs a tall structure that was near, perches and waits for the last vampire to show up. When the vampire arrives and sees the grave has been messed with, he spots the knight and starts attacking. The knight, using the high ground advantage, barely manages to kill the last vampire.

The passing knight continues his travels, but eventually returens to that area and establishes a family, who then proceed to pass down the vampire hunting secrets through generations.