By Brian Lowry

CNN

(CNN) -- The 20th anniversary of Princess Diana's death is an irresistible TV milestone, one that will be recognized in an assortment of programming. HBO seeks to stand apart from that wave with "Diana, Our Mother: Her Life and Legacy," an intimate but gauzy look back, its most salient aspect being that her sons, William and Harry, participate in the reminiscing.

HRH Prince Harry and HRH the Duke of Cambridge were just 12 and 15 when their mother died in a car crash at age 36, and the former describes the loss as "still raw." One needn't be obsessed with the Royal Family to appreciate their grief, especially under such tragic and unnecessary circumstances.

Yet hearing the sons talk about how much they loved their mum, and vice versa, as they flip through a family album is about as deep as "Diana, Our Mother" chooses to get for a sizable portion of this 65-minute documentary, also airing on the UK's ITV. There's a sense that in exchange for this impressive access, the producers felt obligated to construct a film as soft and hazy as a Hallmark Channel movie, complete with the maudlin musical score.

Diana's friends and her brother are equally effusive -- talking about how the Princess of Wales "could light up a room," as Lady Carolyn Warren recalls, and her empathy. Thankfully, that gives way to a richer look at Diana's good works, including her activism involving homelessness, AIDS and Bosnian landmines.

"Diana, Our Mother" is most effective when dealing with such concrete issues, and perhaps inevitably, when it frames her life through events that people remember, as mediated by the images shown on television. There's her seemingly storybook wedding in 1981, juxtaposed with William's 30 years later. And there are shots of people publicly weeping after her death, reflecting its emotional impact -- not just in England, but around the world.

Produced and directed by Ashley Gething, the documentary touches upon the intrusiveness of the paparazzi, who dogged Diana after her separation from Prince Charles. Prince William speaks of the press having "lost its sense of decency," coupled with footage of Diana being hounded and politely but firmly pleading with photographers to respect her children's privacy.

More of that would have given this stately production greater resonance and urgency. As is, Diana's life is presented in the most flattering terms, and her legacy is ultimately embodied by her sons and the lessons they took to heart regarding selflessness and generosity from her example.

The legacy is certainly admirable. The documentary, for all its good intentions, is somewhat less so.

"Diana, Our Mother: Her Life and Legacy" premieres July 24 at 10 p.m. on HBO.