The opening shot of “Weapons” is of a young man enjoying a hamburger at a fast-food joint, facing the camera, clearly having a pleasant day. The scene is in slow-motion, peaceful, almost dreamlike. Then an act of alarming violence occurs, shocking the audience and setting the tone of the film. The reverie is over.

There are many things to like about “Weapons,” from 27-year-old writer-director Adam Bhala Lough, but unfortunately, there are just as many things to dislike. I could dismiss it as yet another exploitative, trying-to-be-significant urban drama about messed-up teens, and a bad one at that — but I can’t ignore that shockingly effective opening shot, the naturalistic acting that renders most of the characters so believable, and the fascinating racial undertones.

Everything in the film takes place before the opening shot, showing the cycle of violence that led to it. It’s a hot summer day in an unnamed Southern city, and Sean (Mark Webber) is back from his first year at college. His loser friends — swaggering alpha male Jason (Riley Smith) and pimply amateur filmmaker Chris (Paul Dano) — are right where he left them, smoking pot on the couch and talking vulgarly about sexual conquests both real and imagined.

Later, while playing basketball in the park, Jason is shot. The story leaps backward then, showing the same afternoon from the perspective of the three black teens responsible for it: responsible job-seeker Reggie (Nick Cannon), his friend Mikey (Jade Yorker) and Mikey’s little brother James (Brandon Smith). It seems Jason raped Reggie’s sister, and her brother and his friends wanted vengeance.

But there is more to the story than that, which we discover as we go further back in time, to the party the night before. One of the nifty tricks Bhala Lough pulls off is shifting the point of view regularly, replaying events from other angles and offering more insight into the complex world of these directionless, angry teenagers. The wandering, documentary-style camerawork enhances the effect.

It is also very interesting how racially integrated the characters are. The violence is generally white-vs-black or black-vs-white, but that is incidental: Race is never the reason for the tension. Reggie didn’t care that his sister was dating a white guy, and the reason Jason was embarrassed to tell people he was seeing her wasn’t that she was black, but that he already had a girlfriend. In fact, race is never mentioned at all. The characters, white and black, are all equally loathsome and trashy, and Bhala Lough goes to some lengths to establish parallels between them that transcend color.

So much for the film’s praiseworthy elements. Much of what transpires is vile and unpleasant, a litany of offenses with no attempt to explain what has made these kids this way. Some of the scenes feel honest and real, but most are full of false macho-teen bravado, with dialogue that sounds like what happens when high school drama students improvise scenes about Important Issues.

It is a shame, because some of these young actors are talented. There just isn’t any point to all this. It reminds me a lot of “Alpha Dog,” only less artful and less meaningful. (If you thought “Alpha Dog” was useless, as many did, “Weapons” will displease you even more.) The filmmaker and his actors have skill. What they don’t have is something to say.

C (1 hr., 25 min.; Not Rated, probably R for a lot of harsh profanity, some graphic violence, strong sexual dialogue.)