Of all the tribal rituals of the Cannes Film Festival — the stars’ swanning climb up the red-carpeted stairs, the swimsuit-clad ingenue cavorting in the surf, the tinkling music from Saint-Saens’s “Aquarium” that precedes every screening — the most colorfully bizarre is the Hijack-by-Hype, wherein a movie of uncertain merit and questionable provenance (and isn’t even in the festival) temporarily upstages the tonier films on offer.

This year’s case in point: “Unlawful Killing,” a documentary about the death of Princess Diana that began to stir up controversy even before it got here. The film, directed by Keith Allen — father of the pop singer Lily, actor in “Trainspotting” and “Shallow Grave” and notoriously colorful off-the-cuffer — earned global disapproval for including a graphic image of the aftermath of the car accident that took Diana’s life in 1997, the details of which have historically been distorted in the interest of taste.

The prematurely offended need not have worried: The photo does appear in “Unlawful Killing,” but only for a moment, and within the legitimate context of Allen’s claim that Diana received tardy and inadequate care immediately after the wreck and that a more timely response would have saved her life.

A reasonable conclusion. But “Unlawful Killing,” which is part of the Cannes “Marche du Film,” or Film Market, and played here Friday to a packed house of buyers and critics, will surely raise hackles for the additional incendiary accusations Allen levels in the film. These include allegations (all of which have found no purchase in official inquiries) that Diana was murdered, most likely by a cabal involving the royal family, the political establishment and the secret services; that she was killed because she was threatening the British arms industry with her work against land mines; and that the inquest into the death, although the longest and most expensive in British history, was little more than a coverup in which the media were either lazily or cynically complicit.

Oh, and the paparazzi were framed.

Actually, that last conclusion wasn’t reached by Allen but by the royal inquisition jury, which found that Diana, her companion, Dodi al-Fayed, and their driver, Henri Paul, were the victims of an “unlawful killing” due to vehicles following them at high speeds. (The occupants of those cars, especially a mysterious white Fiat Uno, were never firmly identified.)

One explosive — and true — fact that Allen never raises in “Unlawful Killing” is that the film was financed entirely by Fayed’s father, Mohamed, who has long harbored suspicions that his son was the victim of a racist royal conspiracy.

Allen’s failure to disclose Mohamed Fayed’s involvement profoundly troubled the journalists who gathered at a crowded news conference after the film, which most of them criticized for failing to provide any new material. But Allen — pugnacious and fit at 57 — simply could not see the problem. Professing not to know how much Fayed gave him ($4 million, Fayed’s spokesman volunteered) or whether his investor also bankrolled the Cannes campaign (do the math), Allen claimed that informing viewers of his financial ties to someone with such a stake in his conclusions “may have interrupted the flow of the story.”

And anyway, he added for goading measure, “there are a tremendous number of films coming out of America and all over the world that are financed by the Mafia, and there’s no reference to [that].”

Fervid buzz of enterprise

It all played out as a slightly tawdrier spectacle than beach babes and be-gowned leading ladies, to be sure. But the “Unlawful Killing” ballyhoo — which drew correspondents from the world’s leading networks, papers and magazines — is also part of the swirl at Cannes. The Palais des Festivals, where most of the action takes place, is a gleaming temple to the cinematic art; meanwhile, not far below the premieres and parties, the moneylenders can be found buying, selling and shilling.

During Cannes, the hive-like basement floor of the Palais hums with the fervid buzz of enterprise of a 24-hour casino or the industrial arts pavilion of a Midwestern state fair. Crammed with kiosks and booths advertising everything from 3-D television systems to developing labs to the benefits of shooting your movie in Buenos Aires, the Market’s trade show seems not just floors but light years away from the festival proper.

Unlike the filmmakers upstairs, who were invited, the film professionals downstairs all pay to be here — the better to pick up additional financing, distribution or an ancillary rights deal.

It may not be pretty. But every once in a while, celebrity, controversy and canny ballyhoo converge to lure even the sniffiest critics to the Market.

“The whole purpose of coming to Cannes is to sell the film, not to publicize the film,” Conor Nolan, Fayed’s spokesman, said outside the packed “Unlawful Killing” news conference.

Either nobody told him that, here at least, they occupy the same glittery-and-grubby double helix, or the absent Fayed has hired an exceptionally shrewd team to navigate it.