We know D.C. Get our free newsletter to stay in the know.

Mitt Romney’s campaign manager may be relieved to hear that September Dawn, an account of a 19th-century massacre that’s been attributed to fanatical Mormons, likely won’t grab much of an audience—it’s a disappointingly conventional Western melodrama. The film is hobbled by a generic script and stilted dialogue—adapted by writer-director Christopher Cain and co-writer Carole Whang Schutter—that distracts from the tale’s believability. Yet for all its clumsiness, the movie has a disturbing power.

“Inspired by actual events,” this is the tale of how 120 settlers were slaughtered in southern Utah on Sept. 11, 1857. Some of the facts come from the confession of John D. Lee, a Mormon who was the only person convicted for participating in the bloodbath, but much of the scenario is Hollywood boilerplate. The screenplay accents the horror with a commonplace device: a romance between members of two feuding communities, in this case a Mormon and a “gentile.” Bishop Jacob Samuelson (Jon Voight), a fictional character, is suspicious when a wagon train arrives in the area he governs. But his handsome, sensitive son, Jonathan (Josh Hartnett look-alike Trent Ford), is drawn to the newcomers, especially pretty, plucky Emily (Tamara Hope). As the young people pledge their eternal love, Lee (Jon Gries) and Jonathan’s father plot the death of these interloping “children of Satan.” The polygamist bishop is particularly upset that one of the women wears pants; as Jonathan concedes to Emily, “a lot of things bother him.”

According to this version of the events, Lee first incites the local Paiute Indians to attack the wagon train. After the Indians are repulsed, the Mormons arrive as rescuers, convincing the settlers to give up their guns and separating the men from the women and children. This Auschwitz-like arrangement facilitates the butchery, depicted in the customary array of quick cuts, fast pans, and slow-motion blood spurts, all cued to William Ross’ tritely elegiac score. In a last-minute appeal to the Titanic audience, the movie closes with a Lee Ann Womack ballad that emptily promises “love will still be there.”

September Dawn doesn’t conjure love with any conviction, whether it’s charting Jonathan and Emily’s doomed romance or noting the migrants’ (somewhat implausible) preference for New Testament kindliness over Old Testament fury. Hate, however, the movie gets right. A comprehensive account of the Mountain Meadows Massacre will probably never be possible, and some argue that the Mormons got a bum rap. Yet the film’s depiction of religious extremism is chillingly believable, largely because of the characterizations of the grimly fatalistic Samuelson and his portentous superior, Brigham Young (a fierce Terence Stamp). Convincing themselves that the settlers are somehow implicated in the death of Mormonism founder Joseph Smith, Samuelson and Young preach a doctrine of “blood atonement”—essentially, that murdering sinners is a form of kindness. This sanctimonious frenzy may not be true to what happened in 1857, but it’s fair to the mind-set of religious cults—and entirely relevant to the age of global jihad.