Natalie Portman plays the title role in “Jane Got a Gun,” the rare Western with a heroine. Jane is an independent woman with a complicated past who has spent years running from a gang of vengeful outlaws. Now they’re coming to wreck the home and family she has built — but Jane decides it’s time to take a stand.

I made that sound pretty good, right? That’s the movie I wanted to see, anyway, but “Jane Got a Gun” fits that description only technically. A Western with potential for feminist table-turning and old-fashioned violence, “Jane Got a Gun” is a major letdown. Despite being co-produced by Portman, the movie sells her character short at nearly every turn.

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That may be less about sexism on anyone’s part and more about director Gavin O’Connor (“Warrior”) and his three screenwriters pulling each other in different directions. Initially, Jane seems like a gunslinger in petticoats. When her husband, Bill Hammond (Noah Emmerich), arrives home riddled with bullets, it’s Jane who extracts the slugs, cauterizes the wounds and rides into town — dressed top to toe in killer black — to buy a regiment’s worth of ammunition. At this point, things seem promising.

Then come flashbacks to Jane’s girlhood romance with Dan Frost (Joel Edgerton, a co-writer), who got whisked away by the Civil War but returned too late to reclaim Jane’s heart. Still, Dan agrees to help defend her home and her new husband. Once this sudsy love triangle surfaces, Jane shrinks to damsel status while Dan — with his military training and broken heart — takes over the movie.

Ewan McGregor almost literally twirls his mustache as John Bishop, a gentleman villain with a black goatee, but it’s never clear what he’s all worked up about. The writers add several plot twists to hold the story together, but “Jane Got a Gun” often seems to be plugging its holes as desperately as Jane plugs her husband’s.

All other problems aside, what really kills this movie is that it’s slow as molasses, stretching 98 minutes into what feels like two-plus hours. In the end, “Jane Got a Gun” doesn’t pack much heat.