“Short Term 12” is what people like to call a “little” film. It has a small, mostly unknown cast. It takes place in five or six nondescript locations. Stripped-down production values and slightly shaky camera work give it the look of a low-budget documentary.

But this surprising little sleeper will give you more big feelings in its first 15 minutes than every one of last summer’s blockbusters combined. It tugs your heart out, then tenderly puts it back, all without losing a vibe so realistic it’s astonishing.

Grace (Brie Larson, “United States of Tara,” in her first lead role) runs a group home for troubled teens who are only supposed to be “short term,” but sometimes end up staying years. As she and boyfriend/fellow staffer Mason (John Gallagher Jr., “The Newsroom”) wrestle demons with kids who have been abused, rejected or given up on, they’re also dealing with their own unresolved personal issues.

Such built-in drama easily could have derailed the story into a string of histrionic Lifetime Movie clichés. But director/writer Destin Cretton, who adapted the screenplay from a short he made in 2008 based on his own work at a teen care facility, draws such honest, natural performances out of his cast that the empathy is effortless. Keith Stanfield is subtly heartbreaking as Marcus, who is about to age out and must leave the home. Kaitlyn Dever is a raw walking wound as Jayden, the new girl harboring secrets.

It’s clear why the film picked up both audience and grand jury awards at SXSW, and mystifying that it was rejected by Sundance, where the short version had previously screened. There are “bigger” movies opening this weekend, but this is the one to choose if you want to be reminded of why we go to the theater in the first place — to feel something that lingers beyond a momentary thrill that’s gone before your popcorn.