Natalie Portman is the putative headliner in “Vox Lux,” an ambitious but ultimately ungraceful meditation on pop superstardom that spans decades, awkwardly weaving themes of school shootings, terrorism, obsessive fandom and post-traumatic stress into the psychological portrait of a singer whose career was born of tragedy.

But she is not the film’s real star.

Like a true rock diva, Portman shows up late in the show, only after the audience has been kept waiting and waiting, entertained by a first act that turns out to be far more satisfying than the main event. Playing the 31-year-old Celeste, an abrasive, troubled and substance-abusing arena-pop performer with a controlling manager (Jude Law), a retinue of sequin-and-spandex-clad backup dancers, millions of fans and a generic-sounding catalogue of wannabe-empowering songs (written by the singer/songwriter Sia), Portman arrives on-screen 45 minutes after the opening credits. Ultimately, she gets upstaged by Raffey Cassidy, who plays her character’s 14-year-old self, in the film’s far more mesmerizing and unpredictable prologue.

That prologue opens in 1999 Staten Island with an act of horror that has, since then, become disturbingly common: Celeste is shot in the neck by a classmate, who goes on to slaughter a roomful of high school music students, and himself. From that act of violence, which includes an interaction between perpetrator and victim that is at once shocking and unexpectedly touching, the career of the 14-year-old Celeste (Cassidy) begins. After recovering from a spinal injury, she and her older sister, Eleanor (Stacy Martin), perform a spontaneous, heartfelt song at a candlelit memorial service. Their tune, with one lyric slightly tweaked — changing the word “I” to “we” — becomes an anthemic hit for a nation in search of healing.

And the rest, as they say, is history — not to mention a cliche. Celeste goes on to becomes a household name, and Eleanor, who writes her songs, a Staten Island homebody.

But it’s the prehistory here that fascinates. Paired with Law’s fast-talking and avuncular manager, who is never named, the duo rides a rocketing trajectory that will be familiar to anyone who has seen “A Star Is Born.” Jetting to Sweden for a recording session and to Los Angeles to shoot a music video, Celeste finds time to grow up a little in between, briefly entertaining a fling with a fellow musician. These scenes avoid melodrama, with the actor-turned-filmmaker Brady Corbet crafting a story that, at least in its first act, feels provocative and scrupulously unhackneyed.

Then Portman appears, in a scenery-chewing turn that is meant to be a tour de force — and some will surely call it that — but that feels less authentic than histrionic. Most inexplicable, the 31-year-old Celeste feels utterly unconnected to the 14-year-old character played by Cassidy, who also does double duty as Celeste’s teenage daughter, Albertine. Those scenes not only highlight the tensions between mother and daughter, but also the resentment between sister and sister: It turns out that Eleanor stayed home to raise Albertine, while Celeste toured the globe. Sparks fly between the two women, generating heat but no light.

You may find yourself asking: Where did the other Celeste go? It’s not just a question of physical casting, but acting choices, including a prominent outer-borough accent that Portman affects, but that Cassidy never uses. It’s unclear what has happened to make the adult Celeste — angry, needy, emotionally abusive — so different from her sweeter adolescent self, even allowing for the effects of trauma and the toll of celebrity. A narrator (Willem Dafoe) fills in some of these gaps, but not enough of them.

More important, where did that other movie go? “Vox Lux,” which takes its name from Celeste’s sixth album, a collection of forgettable power-pop dross, never does anything with its most intriguing themes: the commodification of tragedy and the hollowness of fame. Maybe it’s asking too much to expect Corbet to answer the questions he has raised. Fair enough. But in the climax of this disappointing film, an extended concert scene that features Portman going through a series of awkward, perfunctory dance moves against a backdrop of projected text — “Baby,” “Avec,” “Cash” — the movie doesn’t even seem to have the energy to say anything interesting, let alone something that makes sense.

R. At area theaters. Contains crude language, some strong violence and drug use. 112 minutes.