How does the historical nightmare of slavery inhabit the present day? “Antebellum,” the feature-length debut of directing duo Gerard Bush and Christopher Renz, tackles this question, once explored with originality and nuance by Jordan Peele’s “Get Out,” with shallow simplicity. For all its ideas—the existential threat to white supremacy posed by Black success, the lingering effects of intergenerational trauma, and the insidious romanticization of the Confederacy— “Antebellum” contains symbols rather than people and intrigue without meaningful discovery. That isn’t to say its a boring thriller. The supernatural conceit, which straddles modern existence with life on a Southern Civil War-era plantation, is provocative and alarming. But simply reaffirming what we think we know can only be so effective; and done without regard for complexity—a mere rehash of the crudest racial fantasies and nightmares—the result is bland and aggravating.