Superman, that boring lug, has thankfully flown the coop. Writer-director Thomas Bezucha’s Western thriller “Let Him Go” reunites Diane Lane and Kevin Costner, who as the Kents were easily the warmest presences in the otherwise dully dour “Man of Steel.” They are together again in “Let Him Go,” this time front and center but likewise retired and living close to the land. And the pairing is just as good. Both Lane and Costner, direct and earthy performers from the start, have only added depth with age. As long-married Montana ranchers in “Let Him Go” (in theaters Friday), they’re basically the platonic ideal of an old-fashioned, homespun Americana. They could sell you a mountain of jeans if they wanted to. In Bezucha’s film, adapted from Larry Watson´s novel, they are Margaret and George Blackledge, and the opening scenes are of ominous harmony. Their son James (Ryan Bruce) returns from the fields for breakfast, turning the radio on and dancing with his wife, Lorna (Kayli Carter), and their newborn. If there’s a trace of discord, it’s in a moment where Lorna puts her son under water too hot, leading Margaret to take the child with what probably isn’t her first reproachful glance at her daughter-in-law. It’s an early sign: “Let Him Go” might be set on familiar, tense Western plains, but it’s about mothering. In a series of fast-moving events, James is killed in a horsing accident. Bezucha cuts immediately to three years later, when Lorna is remarrying in a small, joyless ceremony. Soon after, Margaret sees the new husband Donnie (Will Brittain) striking both mother and son on the street. Before she’s had much time to respond to it, she finds the family has unexpectedly moved out of their new apartment. Margaret starts packing up the Chevy station wagon. She’s going to look for them, she says. George, a former lawman, gets in. At this midway point, “Let Him Go” is good. As two childless, grieving figures traversing a Western landscape, the Blackledges seemed carved out of an elegiac mythology. They are trying to beat back the loss they feel creeping over their lives. “Sometimes that’s all life is, Margaret,” says George. “A list of what we’ve lost.” It’s a impression made more powerful by Lane and Costner, both of whom are returning to a genre they’ve visited before but from a different standpoint in life. (Costner is more identifiable with Westerns thanks to “Dances With Wolves,” “Wyatt Earp” and others, but don’t forget Lane in “Lonesome Dove.”) Their search takes them into North Dakota where the dark reputation of Donnie’s family, the Weboys (pronounced “wee-boys”), begins to surround them before they’ve even crossed state lines. Finding them at all seems uncertain, but they manage it, and in a menacing dinner, they meet the Weboys. Their clan, too, comes with a formidable matriarch in Blanche (Lesley Manville, bringing dramatic flare to an otherwise restrained film), a much more violent and sinister corollary to Margret. But the collision of the two families is managed awkwardly. The Weboys, which include a grinning father (Jeffrey Donovan) and two other brothers, never come through clearly. It’s unclear what they’re about, except that they’re bad news, and their cruelty come off as strange, extravagant gestures in uncertainly defined roles. What seems set up for a slow-burn showdown between rival grandmas dissipates in blunt shoot-outs that sidelines Lane’s Margaret at just the wrong time. It’s Margaret who leads the charge, and it’s Lane’s movie even when the film momentarily forgets it. “Let Him Go,” a Focus Features release, is rated R by the Motion Picture Association of America for violence. Running time: 108 minutes. Two and a half stars out of four.