The writer and director Lynn Shelton specializes in low-key comedic parables featuring middle-class white Americans approaching middle age. Her characters aren’t quite hipsters or ex-hipsters, but they do pride themselves on being plugged in culturally (as in 2009’s “Hump Day,” whose straight male subjects seek to affirm their progressive bona fides by making gay porn together). They’re the kind of people whom you can envision listening to “WTF,” the podcast hosted by the wryly dyspeptic comedian and actor Marc Maron.

That’s only one of the ways that Maron’s presence in the lead role of Shelton’s latest film, “Sword of Trust,” makes sense. Shelton has been a “WTF” guest, and has directed Maron on television. And the opening scene shows Maron as Mel, a pawnshop owner, gently puncturing the expectations of a young man selling an “antique” guitar. If you’re familiar with Maron at all, you’ll get the impression that the role was constructed for him. Mel is like Maron with smoother edges: erudite, common-sensical, compassionate and cleareyed, but with a running undercurrent of sarcasm that could turn into a torrent.

Which isn’t to say that Mel is not prone to end-of-the-rainbow delusions. When Cynthia and Mary (Jillian Bell and Michaela Watkins) come into his shop, seeking big bucks for a Civil War sword whose “documentation” supposedly demonstrates that the Confederacy actually won the war, Mel scoffs hard. But his internet-addled employee Nathaniel (Jon Bass) digs up some fringe characters online who say they’ll cough up tens of thousands of dollars for items that prove that the South prevailed. And so the four establish an alliance that drops them down a rabbit hole, starting with a long ride in the carpet-lined rear of an old mail truck.