Only in a spirit of excess cruelty would one refer to NBC's movie "The David Cassidy Story" as a big nothing about a big nobody. Besides, neither the nothing nor the nobody is really very big in the first place.

What we have here is another self-aggrandizing, slightly vindictive show-biz biography, and an extremely authorized and flattering one, since Cassidy himself served as an executive producer.

Most of the time he comes off like the sweetest doe-eyed dearie you'd ever want to meet, a young man perpetually poised to help old ladies cross streets or munificently present his puka shell necklace to the president of the David Cassidy Fan Club--a worldwide organization that in its heyday apparently would have dwarfed the population of China.

(In the interest of scrupulous accuracy: We made up the old lady and did a guesstimate regarding the Chinese, but the puka incident, according to the film, actually happened!!!)

"The David Cassidy Story," tonight at 9 on Channel 4, marks yet another attempt to tap into the nostalgic yearnings of baby boomers and, because reruns of "The Partridge Family" continue unabated, boomer offspring, too. Back in the '70s, that high point of the century for American Bad Taste, young Cassidy was a major mega-throb who could make girls scream, swoon, pant, perspire and even, in one case documented in the movie, keel over with a heart attack.

Structurally and "artistically" (a word that can just barely be applied to projects such as this), the film is a mess--sloppily scripted, shot and slapped together--but that's barely relevant. The key component is voyeurism, a chance to get a supposedly authentic peek backstage at the life of a onetime pop idol. We love to see 'em rise, we love to see 'em fall, and we love to see 'em crawl out of the sewer and, with great difficulty, get back on their feet again.

Americans love success stories, but in the past few years it's begun to seem that they love re-success stories more. Popular cable shows like VH-1's camp-a-ramic "Behind the Music" chronicle case after case of individuals or groups who soared to the top of the charts, slid back off them again--usually with the help of drugs and booze--and then staged miraculous recoveries.

Unfortunately, by the standards of this now ultra-voguish genre, David Cassidy's life hasn't been particularly traumatic or dramatic. His major problem appears to have been that his daddy, actor-singer Jack Cassidy, didn't love him. Jack is played with an ugly sort of gusto by Malcolm McDowell, and is depicted as the kind of show-biz dad insanely jealous of his son's fame and capable of telling him things like, "You're just the flavor of the month. You know that."

Some of the more colorful and deplorable rumors about the elder Cassidy--and there are lots of them--don't get covered in the film. Here he's just a vain ham with a desperate need to be worshiped, whether by an audience or a hooker. After he dies in a fire, son David goes into a lengthy pout that takes up the final third of the film. Even though Papa was a louse, David needed his approval. In a touching fantasy sequence, he finally gets it.

David Cassidy's career, which never amounted to much in either the acting or the singing department (though Lord knows millions of impressionable girls were under the impression that it did), is treated with much reverence. The film opens with this caption on the screen: "London 1974--The Farewell Concert." Oh, "The" farewell concert! Then it flashes back to Broadway of 1956 for a portentous moment with Pops, and then it flashes forward again to Beverly Hills in 1968, where David somewhat reluctantly auditions for a role in "The Partridge Family," a simpering sitcom about a family pop group that tours the land in an oddly painted bus.

Shirley Jones, Cassidy's stepmother, played his mother on the show. Throughout the movie, people are mistakenly calling Jones his "mother" and being corrected by David himself or his agent. Oh the travails and tortures that fame can bring! No wonder David finally tells a pal, "I can't handle it anymore. It's too much pressure. I gotta get out!"

Yes, having screaming teens mob your limousine must be horrifying. Among the other milestones depicted in the film is a Rolling Stone interview in which Cassidy tried to change his image from that of a bubble-gum airhead. He posed for a slightly notorious cover photo in which a patch of pubic hair could be glimpsed; NBC's version is milder.

Cassidy is portrayed throughout as polite and gracious to people, and that includes "Partridge Family" co-star Susan Dey, who according to the film confessed her passionate love to Cassidy after the wrap party for the final episode. Later his heart is broken by a girlfriend whom he promises to take to meet Paul McCartney; before they can visit the Beatle, she tells Cassidy she's engaged to be married.

It's at "London--The Farewell Concert" of "The" World Tour that a young woman with a heart problem dies while screaming orgasmically. Cassidy goes into a depression and takes Valium. Ominously he announces to a friend, "The spotlight has left the building." And yet one can't help suspecting that somehow, by some miracle, through the grace of the Almighty, David Cassidy will live to sing again. Not that his singing was ever worth listening to in the first place.

Andrew Kavovit, a soap opera actor, does a good job playing Cassidy, though he sometimes looks more like a young Regis Philbin. McDowell gives the movie's only real performance as Cassidy's nasty father. Dey Young is not pretty or ingratiating enough to be convincing in the role of Shirley Jones. Cassidy's real-life brother Shaun, arguably even more of a cutie-pie than David, is not mentioned nor portrayed. That seems odd. Another sibling, Patrick Cassidy, is listed in the credits as having been played by an actor, but your normally attentive critic seems to have missed it altogether, but he certainly recognized the real David Cassidy, who does a cameo near the end.

It's hard remaining attentive during "The David Cassidy Story." It's not that easy even remaining awake. One encouraging thing, though: It could be that the schlockmeisters of Hollywood are finally reaching the bottom of the barrel when it comes to portraying folks who've put in time at the bottom of the barrel. "The David Cassidy Story" is fitfully entertaining in some numb, vacuous way, but you'd have to be sky-high on silly pills to consider it even remotely good.