This startling debut feature by writer-director Trey Edward Shults finds horror in the mundane and despair in the bosom of a loving family. The title character (Krisha Fairchild), an older woman whose lapdog is her only friend, arrives at her sister’s house to celebrate Thanksgiving with their extended clan, and the supremely awkward embrace she shares with her estranged college-age son (Shults) communicates a history of savage resentments. Shults permits an avant-garde score of blips and bleeps to dominate the soundtrack during the routine family activities—cooking, chatting, word games, horseplay—and his bold foregrounding of the music at certain points in the action may strike you as either expressive or excessive. I’m inclined toward the latter, but only because the story is so surely unfolded, and its denouement so brutally heartbreaking, that the movie would have connected with no music at all.