“It might be the best song ever written in the history of songs.” So says Anne Hathaway’s character, the titular pop singer in David Lowery’s new film, “Mother Mary,” about a track of hers that you never get to hear. Whether or not you’re inclined to believe her, you can’t prove her wrong. In the course of the movie, Mary repeatedly tries to play a demo for Sam (Michaela Coel), her former costume designer, but Sam refuses to listen to it. As the atmosphere thickens—for much of the film, we stay with the two women in Sam’s atelier, excavating old hurts from their wrecked relationship—the song continues to linger in the air, exerting a pressing negative force. The only thing you know about the song is that it is called “Spooky Action.”

“The best song will never get sung.” So sang Wilco’s Jeff Tweedy more than two decades ago, in a track about a fictional rock band. He was making a joke about rock snobbery—his statement is the logical end point of the hipster’s search for evermore obscure tunes and bands to annex to their private kingdom of refined taste. Still, the point might be more broadly true. Literature is filled with great music that has never been sung or played, at least outside the world of the text. No one has ever heard the Vinteuil Sonata that so captivates the young Marcel in Proust’s “In Search of Lost Time,” or the piping of Kafka’s mouse singer Josephine, or the haunting song that the protagonist of Jean Rhys’s “Let Them Call It Jazz” hears in jail. (One senses that things are more complicated with poetry. A poetic work such as Nathaniel Mackey’s “Song of the Andoumboulou” both invokes its titular song and, over its thousands of lines of mythic sprawl, becomes coextensive with that song.) The notional songs work their magic at an unbridgeable distance; all you can catch is a stray spark or buzz.

The movies are more willing to indulge our curiosity. Unlike musical theatre or opera, where the music often advances the plot or at least has some kind of narrative content, the cinema of pop-and-rock stardom extrudes fake songs incidentally, almost as a waste product. Sometimes this leads to performances that stretch the limits of credibility. Could anyone who heard the title song in “Marry Me” (2022), performed onscreen by Jennifer Lopez and Maluma, really believe that this anodyne slab of synth pop could ever top the Latin charts, as it apparently does in the film’s world? Because you can hear the song, you can judge for yourself, and you may find yourself feeling differently from the scores of cheering extras in the audience.

Then again, an especially good fictional song can come to feel more real than its story of origin. Lustra’s pop-punk cuckoldry anthem “Scotty Doesn’t Know” has detached from the raunchy teen comedy “EuroTrip” (2004) and taken on a life of its own, as has the impossibly infectious title song from “That Thing You Do!” (1996). Such a fate seems appropriate for two films that are largely about the pop song’s twin qualities of endless iterability (“Scotty Doesn’t Know” becomes a worldwide phenomenon and follows the poor sap it was written about everywhere) and absolute singularity (“That Thing You Do!” is the sole hit song by the appropriately named fictional sixties band the Wonders). Sometimes the sheer charisma of a performance is enough to bring a fictional artist’s work into the real-world canon, as in Whitney Houston’s cover of “I Will Always Love You” in “The Bodyguard” (1992) or, to a lesser degree, Lady Gaga’s vocal performance on “Shallow” in the 2018 remake of “A Star Is Born,” which nearly makes you forget the schlock of the song and the melodrama of the movie. Perhaps only with “The Harder They Come” (1972) have a film and its protagonist’s real music, there sung with exquisite defiance by Jimmy Cliff, ascended to the cultural firmament hand in hand.

“Mother Mary” withholds its central song, but it gives us plenty of other original music. Hathaway’s character is an alt-pop superstar in the vein of Gaga and Lana Del Rey, with a dash of Lorde. This is an archetype that has been around for long enough that it could easily be a subject of parody or pastiche, arguably the default modes for the fictional-pop-music film as a genre. The turgid self-seriousness of a pop star who insists that what she is doing is high art, who shushes singing fans at concerts, who turns social-media beef into lyrics: there is more than enough material here for even the laziest parodist. But we will have to wait a bit longer for an alt-pop “This Is Spinal Tap,” or at least be satisfied for now with “The Moment,” the wan Charli XCX mockumentary that was released earlier this year. “Mother Mary” does something more radical: it enlists the musicians and producers who shaped the sound of progressive pop in the twenty-tens to create its fictional music, sung by Hathaway herself. In the hands of Charli XCX, Jack Antonoff, and FKA Twigs, along with collaborators such as the 1975’s George Daniel and the prolific songwriter Tobias Jesso, Jr., Mother Mary’s original songs are more than credible. They are simply the real thing, if not quite on par with these artists’ greatest work. Listening to them is an uncanny experience, as though you are unearthing a suppressed memory of something you heard on a playlist years ago, or discovering a viral trend that somehow passed you by.

Mother Mary is an apt choice of alter ego. The alt-pop of the past fifteen years has delighted in over-the-top personae and world-building. Hathaway’s character, whose government name we never learn, is constantly spinning new mythologies for herself. She shows up unannounced at Sam’s estate in the English countryside, demanding that her former collaborator make her a dress for a special comeback performance following a long hiatus. As the two women pick at their shared history, we learn that Mother Mary, like Taylor Swift, divides her career into eras, each with a distinctive look and sound. (The one visual constant is halo-like headdresses, straight from a Quattrocento Madonna.) Each era seems to reside in a piece of costume, some of which Sam still has on hand. It all comes back to us via flashbacks: someone will brush an old accessory and suddenly we are back there and then, watching a former version of Mother Mary performing. Here she is singing “Cut Ties,” a loping number whose spoken-word intro—which Hathaway tackles with her best impression of Lana in her noir voice-over mode—lifts into a Gaga-style chorus, before cresting in a cacophony of distorted voices. Here she is on “My Mouth Is Lonely for You,” an FKA Twigs-penned song full of burbling synth arpeggios, gamely meeting those breathy high notes.

These performances seem to issue from the other side of a veil; they don’t feel totally contiguous with the film’s here and now. This is partially due to the nature of today’s multimedia pop performances, which tend to present themselves as a whole world apart, self-contained and all-encompassing. But this sense of disconnect is also precisely the problem that Mother Mary is trying to solve. She is out of time, between eras. She needs new myths. To create them, she will have to slough off her old ones: “all the old yous” must be abraded away, as Sam says. And as she puts it elsewhere, more threateningly, “Stitch by stitch, you’re obliterating yourself.” Mary is willing to be annihilated and remade. In fact, in the first half of the film, we come to understand that the source of her star power is not her charisma so much as her blank, twitchy impassivity. She is an icon precisely because she is malleable. “I let you make something of me,” she tells Sam matter-of-factly.

“Spooky Action,” the film encourages you to think at first, could be a rare musical expression of the real Mother Mary. From the way she talks about it, it is clear that it’s a raw and personal song. The closest that you get to hearing the demo is in an arresting dance sequence that she performs for Sam; it is meant to be the choreography that will accompany the song, but all we hear are footfalls and breath. Her movements start out lurching and ugly and grow progressively more frenzied, until she is almost inhuman, demonic, before collapsing in sweaty exhaustion. Apparently Jack Antonoff and Charli XCX had not yet fully settled on a sound for Mother Mary before they saw this scene. Only after they reviewed footage of Hathaway’s “thrashing and jerky” movements, as Charli XCX recounted to Vogue, could they start feeling their way into the music in earnest. In a sense, then, all the music in “Mother Mary” is a distorted negative image of “Spooky Action,” or at least a product of its quantum force.

This force reverberates outside the story, too. Hathaway herself has reported an increase in spooky experiences in her life since joining the “Mother Mary” cast. Lowery has called the film “a doorway into believing, yes, there’s more out there.” In the flashback that opens the second half of the movie, a medium, played by FKA Twigs, appears to tear open a doorway between the film’s world and another. A diaphanous red blob somehow both enters Mary’s body and hovers outside it, pursuing her on tour. Both Mary and Sam say that they have felt its presence, even at a distance. At the film’s climax, they stage an exorcism: Mary carves a gaping wound in her chest, and Sam pulls out a piece of billowing red fabric, like a magician. What previously could have been a shared delusion now takes on a solid form in the world.

This, ultimately, is what the pop song is for “Mother Mary”: it is a form of dark magic, a portal between realms. Another film might want to demystify its spell and show you the creaky machinery under the surface—the bumbling ineptitude of the industry suits, the little humiliations of touring life. For better or for worse, “Mother Mary” really wants you to believe. ♦