When Betting on Zero was shown as at the National Portrait Gallery's 346-seat auditorium as part of last year's Investigative Film Festival, half the seats to the screening of the documentary - a searing expose arguing that the nutritional supplement company Herbalife is a Ponzi scheme - went empty.

According to the festival's organisers, several employees of Herbalife's Washington, DC, lobbying firm, Heather Podesta + Partners, bought tickets in large batches, which were never used. Earlier last year, at the world premiere of the film at Tribeca, paid protesters handed out leaflets to try to discredit the movie, according to the Betting on Zero website.

Why all the fuss?

Because Betting on Zero makes such a strong and effective case that the company does, in fact, engage in shady business practices that it's likely to leave viewers in a state of Documentary High Dudgeon (that brand of cinematic outrage that is not entirely unmixed with a pleasurable feeling of moral superiority).

The chief purveyor of that emotion is Bill Ackman, a hedge-fund manager with a beatific smile and a righteous anger. He has made it his mission to take down Herbalife - and to make a profit - by taking a massive "short" position in the company's stock. (Meaning that his investment firm, Pershing Square Capital Management, will make a ton of money, which he has promised to give away, only if Herbalife shares go down the toilet. Hence the film's title.)

Other testimony in the film (from director Ted Braun of Darfur Now) comes from disgruntled, and deeply in debt, former Herbalife salespeople, many of whom are Latino immigrants seeking the American Dream. This lends the film a pungent topicality.

The most damning evidence comes from Herbalife corporate videos touting the company's business model, in which the most money is made not by selling its products but by recruiting others to do so. It also doesn't help that the Federal Trade Commission has charged Herbalife with "unfair, false and deceptive business practices", as an on-screen title tells us at the end of the film.

There's been an additional development since the Investigative Film Festival screening, and an on-screen title has been added to the film's epilogue. Throughout Betting on Zero, investor Carl Icahn appears as Ackman's chief adversary, arguing that Herbalife is a strong and ethical business, and putting his money where his mouth is: Icahn has invested heavily in Herbalife stock, essentially betting against Ackman by hoping the company will prosper.

Just before the closing credits roll, we're shown these ominous words: "On December 21, 2016, Donald Trump named Carl Icahn special adviser to the president on regulatory reform."

Betting on Zero

• Three and a half stars out of four.
• Unrated.
• Contains nothing objectionable.
• 99 minutes.
• Advance screenings from Friday March 17

Ratings Guide: Four stars masterpiece, three stars very good, two stars OK, one star poor, no stars waste of time.