The message of Peter Heller's novels could be boiled down to: Nature is enthralling, humans. Stop being idiots and pay attention!

Heller has shifted from the meditative, almost poetic tone of his first couple books to out-and-out thrillers that are set in beautiful places where people do stupid things to mess them up. Most of the events of "The Last Ranger" take place in Yellowstone National Park, where ranger Ren Hooper has his hands full dealing with campers high on Fentanyl and parents who Instagram their kids sidling up to adorable-but-deadly baby moose.

Heller's best books, especially his riveting "The River" — in which two young canoers try to outpaddle a vicious thug — have a lickety-split pace and archetypal characters whose behavior makes sense to us partly because he keeps them mysterious, forcing us to fill in their motivations.

In "The Last Ranger," he takes a different tack, using flashbacks to tell us what drives Ren (abandoned by his mother, haunted by a childhood tragedy). There's also a possible romance — with Hilly, a woman who studies wolves (you can tell who you're supposed to like in Heller books because even their names sound like nature).

That means "Last Ranger" doesn't have the narrative momentum of "The River" or his last one, "The Guide." There's a mystery of sorts, as Ren investigates a trap that nearly killed Hilly. We're pretty sure we know the culprit, so "Last Ranger" almost works like a police procedural, if the station house were replaced by meadows and elk jerky subbed in for doughnuts.

"The Last Ranger" has the feel of a series launch but if it is, Heller will need to interrogate the motives of Ren, whose split-second decisionmaking turns him into a vigilante in a way that the book doesn't acknowledge.

On the other hand, throughout the novel there's a sense that good and evil aren't as easy to separate as we'd like to believe. Maybe Heller's point is that the "good guys" are the mountains and the streams and the "bad guys" are all the people who think those things were put here for us.

The Last Ranger

By: Peter Heller.

Publisher: Knopf, 287 pages, $27.