Starring Jessica Chastain. Rated PG. Now playing

It turns out that 50 years of progress can be wiped out in the blink of an eye. Suddenly, we’re back to Miss for an unmarried woman—even one who is the epitome of Pantsuit Nation independence. That would be one Elizabeth Sloane, a Washington lobbyist played by Jessica Chastain with the steely resolve she brought to Zero Dark Thirty, only scarier.

Sloane is the top shark for a venal lobbying crew led by Sam Waterston’s eyebrows. Given her reputation for slithering unscrupulousness, everyone is surprised when she bolts after her company lands the beyond lucrative gun lobby. She’s then recruited by a smaller, more ethical outfit promoting very mild legislation requiring slightly more stringent background checks for gun buyers. This killer lobbyist’s win-at-any-cost methods rankle her new boss (England’s Mark Strong, struggling with his American accent) and potential in-house pal (fellow Brit Gugu Mbatha-Raw, nailing hers). But they’re impressed when she starts nudging Congress towards their side of the bill.

In first-timer Jonathan Perera’s generally clever script, her team (including many familiar Canadians, as this was shot in Toronto) spends roughly equal time discussing political arcana and the obvious. The latter stuff is for the audience, which also expects an antihero like Sloane to have feet of clay, and here it’s a propensity to pop uppers, keep the company of paid escorts (as repped by Girls’ Midwestern hunk Jake Lacy), and use the people around her like pawns.

Perera’s snappy dialogue aspires to Aaron Sorkin-type crackle, and almost gets there, thanks to vigorous direction from veteran John Madden (Shakespeare in Love and the Best Exotic Marigold Hotel movies), working with a top-flight cast, including Michael Stuhlbarg, Allison Pill, Dylan Baker, John Lithgow, and Christine Baranski.

It all unfolds briskly, even at more than two hours—mainly due to Chastain’s utter conviction—although the tale can’t quite decide whether to be a profound character study or a gotcha thriller, full of increasingly outlandish twists. Ultimately, the most preposterous things on view are the film’s ill-timed assertions about ethics and perceived conflicts-of-interests—notions now as quaintly nostalgic in the American capital as an honorarium like Ms.