What’s a mentally slow cowboy to do when he finds himself in Los Angeles? In “Down in the Valley,” he starts sleeping with a teenager, makes a best friend out of her 11-year-old brother, and then ruins their lives.

Harlan Carruthers (Edward Norton) is a wandering cowboy who refuses to accept the strictures of urban life. He won’t drive a car or hold down a suffocating job. Instead, he dresses like a rancher, borrows horses when he needs a reminder of ranching life, and speaks in embarrassing colloquialisms. At one point, when a Los Angelino teen is confused by his presence in the valley and asks him if he’s for real, Mr. Norton is obliged to say: “I think so. Wanna give a squeeze?”

This facade is enough to entertain the bored Tobe (Evan Rachel Wood), a high school student who starts up a relationship with the 30-something rambler. And while Mr. Norton’s sex appeal makes this interest plausible, his awshucks demeanor soon wears thin.

Tobe and her younger brother Lonnie (Rory Culkin) are both desperate for a distraction from the dysfunctional existence they share with their father. For them, Harlan is an appealing diversion. For the audience, he quickly loses his appeal.

As Tobe and Harlan interminably frolic in the beginning stages of their relationship, the audience must put up with him referring to pot as “wacky weed” and telling the 16-year-old: “Most days, I just want to step outside my heart.”

Mr. Norton’s interpretation doesn’t help. The vulnerable toughness that made him a success in films like “Fight Club” works against him here. He overplays Harlan’s naif qualities, and the focus on his thin physique gives Harlan a comical presence that doesn’t always seem intentional.

Ms. Wood, who has shown her abilities in “Thirteen” and “Pretty Persuasion,” is often left spinning her wheels.

It seems she was picked for the role because of the rebellious adolescent she played in “Thirteen,” but Tobe is only a shadow of that character.

By the time the film starts peeling away the realities of Harlan’s character, the exposition is overbearing. Every act and scene becomes overly symbolic. The film spends so much time getting where it’s going that all the audience’s good will is spent by the time the plot catches up. When a gun’s barrel first gleams on screen, it’s clear that the anti-gun lobby will approve the second half of the film.

The filmmakers and Mr. Norton seem enamored with the juxtaposition of a cowboy in downtown Los Angeles. There are scenes of Mr. Norton preaching to streets of driving masses and trotting on horseback through the city streets. By the time this cowboy wanders onto the set of a Western movie being filmed, it becomes a small tragedy that the actors’ guns are only equipped with blanks.

Mr. Norton can be an appealing presence on film, but he’s wasted here. There are beautiful vistas and some good moments in the film, but on the whole, “Down in the Valley” misses the point. While Mr. Norton’s character in “Fight Club” may have shared with Harlan a similar misunderstanding of himself, that character had a nuanced, everyman quality that was compelling up until the denouement. And most everyone has some connection to the oppression of corporate life. But the cowboy come to rescue an egotistic society from itself may be a subject that is most appealing to those trapped in Hollywood. Harlan’s Crocodile Dundee syndrome may have some sway with people tired of their lot in life, but most will find him irritatingly slow. And the rest of the audience has two hours with him to reach that conclusion.

mkeane@nysun.com