Blue Moon by Lee Child

Reacher sits in the back of a Greyhound bus watching an old man asleep in his seat, a fat envelope of money about to slip from his pocket. Reacher's also watching a guy who would like to get his hands on that money. Ain't gonna happen. The old man is grateful and thanks Reacher but turns down Reacher's offer to help him home. Huh? He's vulnerable, frightened, and clearly in big trouble.

Coincidentally (or is it? - it's a Reacher novel, after all), this city has a gang problem. The Albanians vs. Ukrainians. Is that why the man is so scared? Will Reacher sit back and let bad things happen? Or can he twist the situation to everyone's benefit?

"This is a random universe," he says. "Once in a blue moon things turn out just right."