Jack Reacher, as he so often does during his wanderings, finds himself in the middle of something really bad, something that only Reacher is equipped to cope with. (Reacher, as you know if you've read the series, is equipped to cope with just about anything short of the apocalypse or something close to it. Or [HUGE SPOILER] maybe he can't.)
Bolton is home to a prison. On the outskirts of town, on an abandoned Air Force base that no one remembers the purpose of (not even the folks in Washington), a meth-peddling biker gang has set up shop. Their leader's in jail, waiting for his trial, and the only witness against him is under heavy guard. And people start dying, killed by an assassin that the cops believe has slipped into town. They're wrong about that. Reacher figures out who the assassin is about 31 hours too late. (Though I knew it when he should have known it. Maybe I've read too many books.)
There's more going on, too. A drug lord from down south of the border is about to complete a huge deal that involves the abandoned base, and he's flying in. And, well, it's complicated. There's a lot going on.
And then there's the ending. It's bothered a lot of people. Didn't bother me, but I can say no more. You'll have to read the book to find out.