If you’ve ever caught yourself having just too good a time with “Freaky Friday” or “Your Name” or really any entry in the oddly well-populated body-swap genre, here’s a chance to do a little penance. “The Unknown” is Arthur Harari‘s third directorial feature, after “Dark Inclusion” and “Onoda: 10,000 Nights in the Jungle,” and his return to Cannes competition after co-writing Justine Triet’s blistering Palme d’Or-winner “Anatomy of a Fall.” But absent “Anatomy’s” mordant wit, and lacking even the sturdiness of “Onoda,” “The Unknown” makes ponderously heavy going of its switcheroo storyline, which is led by Léa Seydoux sulkily facing down the challenge — to be fair, quite a big one — of embodying a character who feels aggrieved that they look like Léa Seydoux. 

Her counterpart through the murky psychological looking-glass is French-Canadian actor Niels Schneider, here reuniting with Harari after his César-winning role in “Dark Inclusion.” Schneider plays David, a reticent young man with the soulful-hobo air of a Beat poet, who makes a living as an events photographer but whose private passion is a secretive lifelong project, inherited from his father, documenting the changing Parisian suburbs.

His apartment bookshelf is lined with photo albums in which the evolution of various street corners and Montreuil storefronts is shown in then-and-now snapshots, taken decades apart. Observing how a city’s facades change and wondering what that does to the municipal spirit behind them has a parallel with the major makeover David is about to undergo. It is not, however, a terribly edifying parallel, nor one that Harari’s overly cautious screenplay (which is based on a graphic novel he collaborated on with his brother Lucas, and co-written by both Hararis and Vincent Poymiro) cares much to explore.

Encouraged out to a party one night by his gregarious best friend (Shanti Masud), David accepts a pill from a stranger and, almost immediately afterwards, notices a young woman (Seydoux) across the room who is fixing him with a faintly predatory stare. He follows her into a back room where they have wordless, her-on-top, fully clothed sex, after which David blacks out. He makes it back to his apartment, bleary and disoriented, the next morning, which is when he discovers he has somehow been transferred into the body of his mysterious hookup. To all outward appearances, he is now a woman about whom he knows nothing, not even her name. 

It’s an interesting instinct, to drain the body-swap premise of its comic, Steve-Martin-channelling-Lily-Tomlin potential so that it hews closer in feel to something like Kafka’s “Metamorphosis” — a clear inspiration here, albeit shorn of the novella’s absurdist gallows humor. But along with the comedy, a lot of the life has drained out of the concept too. And David has not turned into a giant bug, he’s turned into a human woman (and one he found attractive enough to want to sleep with in the first place), so his subsequent incuriosity about the body he now inhabits seems at best unlikely, and at worst a failure of nerve on the part of a filmmaker determined to intellectualize an inherently biological quandary. 

Perhaps that could work if the film were operating in a more obviously metaphorical or surreal register. But everything from the muted, eyes-downcast cinematography (the director also collaborates with his other brother, DP Tom Harari, returning after “Onoda”) to the characters’ poky apartments and the unromantic suburban locations cues us to invest in “The Unknown” as a what-if scenario that takes place in the real, ordinary world.

Not to suggest that all 137 convoluted minutes of runtime ought to be spent riffing on some variation on “Wow, so I have boobs now?”. But within such an everyday milieu, it’s difficult to believe anyone would be so uninterested in a newly transformed physique — especially one of the opposite sex. And that’s just alone and in private; David appears similarly incurious about all the ways society at large will treat him differently, now that he looks like Léa Seydoux. 

This approach also robs the film of much traction as a trans allegory, which would appear to be its most readily available avenue of thematic exploration, as rather than contending with the gendered implications of now being a heterosexual man trapped in a woman’s body, David’s transformation is treated simply as a mystery to be solved. To that end, there are some ingenious bits of business as he proves surprisingly resourceful in identifying the woman — her name is/was Eva — and eventually tracking down, through a combination of sleuthing and convenient coincidence, the person now occupying his old body.

To his shock, and our confusion, it turns out this is not Eva herself, but Malia (Lilith Grasmug), a different young woman who experienced a similar cosmic calamity following a tryst at a music festival. The pair realizes that there must be others out there, because who knows how far back the chain of body-hopping started?

With Malia-as-David, “The Unknown” actually does find a little allegorical and emotive power. She is entirely miserable in her hated male body — though again, the cruel irony of expressing as much to David-as-Eva, the man whose body it is, is played self-seriously straight. And Malia is also achingly lonesome for her family life, with a sister about to get married and a doting father played, in this headache-inducingly twisty psychodrama’s most refreshing meta-twist, by Romanian director Radu Jude. 

Not to make too much of what is a relatively small (though not insignificant) role, but Jude’s anomalous casting as Malia’s Marcus Aurelius-quoting, crane operator dad does carry with it a current of eccentric vivacity that the rest of the film sorely lacks. And in the film’s one concession to the familiar tropes of the body-swap canon, Malia, in David’s body, does actually approach her father in an attempt, such as we probably all would make, to convince him of her real identity using her intimate knowledge of their shared history.

That this surprisingly moving scene is then revealed to be merely imaginary is another example of the hesitant screenplay’s frustrating tendency not to follow through on the most obviously dramatic consequences of its arable premise. With all due apologies to any real-world sufferers of supernatural body-switching, who perhaps regard the film’s high-mindedness as a welcome corrective to the condition’s flippantly comedic treatment in pop culture more generally, the real unknown of “The Unknown” is the reason behind making a body-swap movie feel so wholly disembodied.