That's a shame, in at least three ways. First, gifted actress Naomi Watts deserves to be in a better movie. Second, Oliver Hirschbiegel, who directed the Oscar-nominated German-language film "Downfall," about Adolf Hitler, somehow, er, falls down here.
Finally, and most unfortunately, an opportunity is lost to dig deeper into a personality that fascinated the world like few others in modern times.
Where does it go wrong? For starters, the filmmakers may have been constrained by a desire to be respectful. It's not hard to imagine why. Diana's two sons are alive, for one thing.
But blame must also be laid on the script. Yes, we know that royals speak woodenly in public. But we're pretty sure they, and the non-royals in their lives, loosen up in private. Stephen Jeffreys' script sometimes sounds like he's unaware how real people chat, flirt, fall in love.
Speaking of love: The film focuses on Diana's nearly two-year affair — a passionate one, by all accounts — with Pakistani heart surgeon Hasnat Khan.
That affair isn't news. However, the film implies that Dodi Fayed, the boyfriend who died with her in that 1997 crash in a Paris traffic tunnel, was merely a minor fling — a fling that was chiefly an effort by Diana to make Khan jealous.
Is this true? Well, it's the contention of a 2000 book the script is based on, Kate Snell's "Diana: Her Last Love" (Snell is an associate producer on the film). Khan has told the British media that he has no intention of seeing the film, and he's sure it got it all wrong.
The film is slavishly devoted to capturing Diana's look and style. Watts wears a prosthetic nose, and she works gamely to capture Diana's coy expressions and body language. And it must be said that the clothes — carefully recreated — look great.
The film starts on that fateful night in Paris, at the Ritz Hotel. As Diana, Dodi and a small entourage enter the elevator to leave after dinner, the scene turns eerily into simulated security footage, hinting at the painstaking investigations to come.
We all know what happens next. But we don't see it. Instead, the movie rewinds two years. Diana is separated from Charles, living in Kensington Palace, heating up baked beans for supper.
In a chance hospital meeting, she encounters Khan (Naveen Andrews of "Lost" and "The English Patient," who might have done a better job with better dialogue.) Soon, the two are having secret trysts.
The movie may not be great, but for some it will be a fine guilty pleasure. Just watching how Diana lived the last two years of her life — or, OK, watching some approximation of it — is not the worst way to spend two hours.
After all: This is Diana we're talking about.