For the first 40 minutes, “Antebellum” looks like a bad Civil War movie.

Everything is too clean, the occasional bit of dialogue sounds too modern for the 1860s and the villains are almost cartoonish in their cruelty.

Those first 40 minutes don’t feel like the 1860s because they don’t take place during the Civil War; they take place in the present.

That’s the kind of plot twist that normally would be considered a spoiler. Directors Gerard Bush and Christopher Renz treat it as such in the way the film is structured.

But the marketing campaign makes it clear that Janelle Monae isn’t playing a 19th century slave, so it seems foolish to hide that fact.

Monae’s Veronica Henley is an author and sociopolitical commentator with a loving husband, a precious daughter and a successful life that is upended when she finds herself cast as a slave in a truly hellish cosplay.

It’s a timely premise for racially charged times. There are a lot of folks who seem to like dressing up as Confederate soldiers and southern belles. It’s impossible to get the full experience without some slaves, and nobody’s volunteering for those roles.

For a twist that most of the audience will know before the movie starts, those first 40 minutes are interminable. That world could have been established half of the time, probably less.

But it’s not like the next 65 minutes are significantly better. “Antebellum” is a well-intentioned mess, a movie that uses racial politics like a bludgeon to deliver its message.

It’s hard to blame Bush and Renz for the lack of subtlety. With a significant portion of the population believing racism no longer is a problem, maybe some people need to be smacked in the face with it.

“Antebellum” does just that. There are moments designed to recall past murders of African-Americans, particularly James Byrd Jr. Confederate flags and Confederate monuments play prominent roles, although the ham-fisted way they’re incorporated into the story detracts from the horror-thriller element. The woman should be running for her life, and instead the movie has her taking down Confederate flags and reading monuments.

It may work as a polemic, but the movie fails as a horror-thriller, and that’s the only chance it has to lure an audience that isn’t already sympathetic to its message.

“Antebellum” is most enjoyable in its happier moments — Veronica’s scenes with her husband (Marque Richardson) and daughter ( London Boyce), her interactions with her friends Sarah (Lily Cowles) and Dawn (Gabourey Sidibe), a relationship guru.

The slavery sequences are just grim (which is how slavery should be presented), but Veronica’s attempt to escape isn’t particularly cathartic either. And the movie hints at a much darker ending than it delivers.

The marketing campaign points out that “Antebellum” was made by the producers of “Get Out.” With the commercial success of that film and a bonafide pop star in Monae in the lead role, it’s hard to believe that Lionsgate is going with a video-on-demand release instead of also putting it in theaters, especially at a time when exhibitors are starved for new films to fill their reopening multiplexes.

After seeing the movie, the marketing decision makes more sense. “Antebellum” mostly will remind viewers how much “Get Out” got right, and that getting it right isn’t as easy as that movie made it look.