One of the themes that recurs through the Old Testament is that the God of Israel isn't only interested in the people of Israel. The God of Israel is willing to act on behalf of people who are not part of the nation of Israel.
This is one of those stories. Naaman is the general for the army of one of Israel's neighbors. He comes down with leprosy, the most terrifying disease of Biblical Israel. When he has no other choice he turns to the God of his wife's slave girl for help.
There are two things about this story that make it interesting:
1. The God of Israel is willing to act, even for someone who has been and might be again the enemy of his chosen people
2. The God of Israel doesn't act in the same way that other gods do. There is no flash of light, no loud noises, no theatrics, just an instruction to go wash in the river.
We see in this story God acting in unexpected ways - that is another one of the themes that recurs throughout Scripture. God is often found in unexpected places doing unexpected things.