You might be tempted to check out “Trespass Against Us” simply because of its cast, toplined by Michael Fassbender and Brendan Gleeson. But before you plunk down money for a ticket, remember that both of them also appeared in the recent “Assassin’s Creed.” Consider Adam Smith’s film their second joint strike, a gritty but rather pretentious tale of father-son conflict in a backwoods British family. Though the locale is obviously different, anyone familiar with the current WGN series “The Outsiders” will recognize the premise. That does not constitute an endorsement.

Gleeson plays Colby Cutler, the head of a clan of outlaws living in trailers outside a town in West England. The group is apparently a part of the Travelers, the Irish gypsies referenced in such past movies as Jack Green’s 1997 “Traveller” and Guy Ritchie’s 2000 “Snatch”—at least if remarks Colby makes to his family late in the picture, in which he says that the key to every problem is to travel—are any indication. He lords it over the encampment like a big bear, ordinarily chummy and certainly protective of mentally impaired followers like Gordon (Sean Harris), a goof who runs about the camp engaging in idiotic pranks, but turning threatening on a dime when his wishes aren’t followed.

At the moment his biggest problem is his son Chad (Fassbender), who follows Colby’s orders but is also gingerly taking steps to move his family—wife Kelly (Lyndsey Marshal) and their kids—toward a more normal life. For example, they send their son Tyson (Georgie Smith) to school, where he learns about evolution—which Colby dismisses as nonsense, just as he does the idea that the earth is round. (This is the source of the picture’s title—Colby sees his clan being victimized by encounters with the world outside.) Colby pretends to be tolerant of Chad’s occasional rebelliousness, though in fact he is not as forgiving as he appears on the surface.

A showdown of sorts will eventually come between the men, but for all the anger Chad might feel toward his father, his animosity toward the local cops—especially officious P.C. Lovage, played by Rory Kinnear—is even greater, and when he finds himself on the run from the law, he has to decide where his loyalties lie. “Trespass Against Us” abandons its grittiness at the close, opting instead for a finish that opts for a nearly magic-realist flourish in which solidarity in the face of a common foe becomes the key.

Fassbender and Gleeson certainly sink their teeth into their roles, even though as contrived by scripter Alastair Siddons there isn’t much meat in them, just a coating of surface gristle. The supporting cast is game as well, but their parts are written with even less nuance. This is Smith’s first feature—his previous work was in music videos and television—and he fails to muster much control over the material, leading to sequences—like a couple of car chases—that are momentarily striking but eventually collapse into chaos. His production team—headed by cinematographer Edu Grau, production designer Nick Palmer and editors Kristina Hetherington and Jake Roberts—don’t do much to give shape to the anarchic piece, either, though the score by Tom Rowlands does at least provide an injection of energy.

One can imagine a powerful, insightful film being made about rifts within the English Travelers, but unfortunately “Trespass Against Us” isn’t it.