Weapons are brandished but rarely deployed in Lynn Shelton’s “Sword of Trust,” a smart, politically barbed comedy about truthiness and consequences.

At various points the characters wave handguns, screwdrivers and, yes, the sword of the title, a Civil War relic that carries a dubious but potentially dangerous history. No one really gets hurt — a character’s words draw more blood than anything else — but you can feel the hostility in the air, and much of the laughter leaves an aftertaste of unease. In this uneven but peculiarly potent movie, the violence of the past looms uneasily over the present.

The sword has been bequeathed to a woman named Cynthia (Jillian Bell) by her grandfather, who lived in Alabama until his recent death. Cynthia and her partner, Mary (Michaela Watkins), were hoping for a more generous inheritance, but this one is not without its lucrative possibilities. They take it to a Birmingham, Ala., pawnshop run by Mel (Marc Maron), who listens with growing skepticism as the women rattle off the sword’s strange backstory: According to some sketchy accompanying documents, it’s a Union Army blade that was surrendered to Confederate forces after a fateful battle and thus offers proof that the South actually won the Civil War.

No one really believes the story — least of all Mel, who is as tough and battered by experience as some of the antiques in his shop — but neither is anyone above exploiting it for personal gain.

It doesn’t take long for Mel’s lazy, good-natured assistant, Nathaniel (Jon Bass), himself something of an internet conspiracy-theory enthusiast, to track down a group of Civil War truthers willing to pay top dollar for “prover items.” Before long, the four of them are negotiating with a guy named Hog Jaws (Toby Huss) and tumbling down a rabbit hole of white-supremacist intrigue, driven by greed but also by a certain perverse fascination about how the lunatic fringe lives.

Shelton, who wrote the script with Mike O’Brien, trusts us to share that fascination. Long identified with American independent cinema’s mumblecore movement, she has a knack for drawing strong, semi-improvised performances from her actors, as she did in her delightfully shaggy 2009 comedy, “Humpday.” She makes that improvisation more thematically explicit in “Sword of Trust,” structuring the plot around episodes in which the characters must perform and deceive others on the fly, essentially making things up as they go along.

To wit: Cynthia and Mary try to sell the sword and its pro-Confederate backstory to Mel and Nathaniel. All four of them try to do the same with Hog. Three of them must downplay their identities — Cynthia and Mary are a lesbian couple, and Mel is Jewish — to fool the knuckleheads they’re dealing with.

At one point the director gets in on the act, turning up in a brief, vivid role as Deirdre, an old flame of Mel’s who tries to hit him up for cash. The details of their romantic history will emerge in due course, but even in this scene’s oblique outline you can sense a near-lifetime’s worth of unspoken resentments and agonies coursing between them.

If Shelton and Maron feel particularly in sync, it may be because they have collaborated often; she has been a guest on his popular podcast, “WTF,” and previously directed him in comedy specials and shows, including the Netflix series “GLOW.” Maron does beautifully shaded character work here as a man with deep reserves of life knowledge and a tetchy know-it-all streak to go with it.

But all defensiveness melts away in his finest scene, which takes place inside a heavily insulated moving van carrying Mel and his friends to an uncertain destination — a potential trap that becomes an unexpected zone of discovery.

Not all of “Sword of Trust” can live up to a moment that emotionally layered. At times the movie seems content to substitute bumbling comedy for deeper insight, and its prodding at politics doesn’t have enough follow-through: The characters seem to understand on a theoretical level that they are dealing with people armed with guns and hate, but what that might actually mean (Mel says “We’re gonna die!” a few times, always a little sarcastically) seems to elude them, as does any sense of their own responsibility in potentially fanning the flames of racist ideology.

More than once you may scoff at the reckless foolishness of the foursome as they descend into the truthers’ lair; it’s a bit like watching horror-movie protagonists in a picture that ultimately has no real interest in being a horror movie.

“Sword of Trust” evokes the specter of American divisions past and present — between North and South, right and left — and suggests that comedy has the ability to disarm them all. It’s a heartening idea, but it could be sharper.

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