Clouds of Sils Maria is essentially three different films, and each one in order is a little bit less interesting. But for a while there, for maybe the first 25 minutes or so, it seems that Olivier Assayas has done it again, knocking out a fascinating character study of life in a constantly plugged-in, celebrity-addicted world, and he does it in what I'd call the most unexpected way possible: by putting his whole first act on the shoulders of Kristen Stewart, late of the Twilight franchise and unlikely to ever outlive the memories of all of us who in those days thought she was just the worst thing.

Unlikely, but it's not impossible, not if she keeps up to at this level of ambition with directors and scripts. Her performance in Sils Maria is the best work she's ever done, and better still, a top-shelf performance by the standards of any actor in any movie. She plays Valentine, the highly-competent twenty-something assistant to middle-aged international movie star Maria Enders (Juliette Binoche), and for that first 25 minutes, the movie is all about her: the juggling of phone calls and calendars, the emotional massaging, the steely mama bear attitude she adopts in relationship to the much older but also much more tetchy and destructible Maria. It's vintage Assayas, anatomising the behaviors and attitudes of technology-driven life, just abstract enough and set in a series of heightened, unreal locations (a seemingly endless train, a glamorous but oddly empty hotel in the Swiss Alps) to avoid seeming like blunt docudrama. And Stewart is our marvelous host through this world, storming through the plot while letting her character's unexpressed emotions linger there, for us to engage with or not as we see fit.

Unfortunately, it becomes clear before too long that this is not the actual film that Sils Maria wants to be. In Switzerland, reeling from the death of her onetime mentor and most beloved author Wilhelm Melchior - she was in town to accept an award on his behalf, before suddenly forced to serve as centerpiece of a memorial service instead - Maria is approached by ambitious pan-European theater director Klaus Diesterweg (Lars Eidinger) with a gimmicky offer: he wants her to play the part of the older woman in Melchior's psychological drama about female homoeroticism that was, 20 years ago, the starmaking role for Maria on both stage and screen, when she played the younger woman. Appealing to her vanity, her sense of history, and her sense of obligation to the newly deceased author, Klaus convinces her to take the part opposite self-destructive, scandal-prone Hollywood starlet Jo-Ann Ellis (Chloë Grace Moretz). And so Maria and Valentine head off to the Swiss valley of Sils Maria, there to study the play in Melchior's own home.

The film that Sils Maria becomes with this development is a hell of a lot less interesting, on account of being a hell of a lot more conventional in its themes about a middle-aged actress confronting herself aging. But even more because, completely uncharacteristically, Assayas's screenplay decides right about now that we are incredible idiots who need to have things explained to us, and so for more than an hour as Maria and Valentine run lines, and Maria hates herself and the universe while Valentine explains, with increasingly little patience, why Maria is beautiful and a genius, the film just plain starts grinding.

It is built on a series of metaphors, mostly in the script but some in the way that Assayas and cinematographer Yorick Le Saux film the Alps and the beautiful home where Maria tries to find her character. And these are not, in the main, very complicated metaphors, nor very original metaphors, neither of which is really all that big of a problem. No, the problem is when Assayas rumbles along and then has his characters deconstruct those metaphors right before our eyes, explaining in small words, and a lot of them, that playing the character she's always thought was a pathetic and weak forty-year-old waste of flesh is making Maria confront her fear of being pathetic, weak, and forty. And the even bigger problem is that Assayas doesn't just do this once; he does it over and over and over. If all you did was remove the scenes explicitly telling us things that other scenes already explicitly told us, you'd cut out 20 minutes of the film in the blink of an eye. There are even meta-layers of this same problem: the script to the play Maria's rehearsing is hard for her to play because it's so overt with its themes. And then she and Valentine start to discuss how those too-overt themes are also the themes of Maria's life, which makes it even harder for her to deal with how overt they are.

It's still better than the aneurysm that the film has when it enters its protracted epilogue, around the day of the final dress rehearsal, and it ceases to be about anything at all but protracting itself out to ruinous, suffocating lengths. But I would not want to spoil things. There's plenty to feel disappointed in throughout that whole middle (the ending, meanwhile, makes me feel outright hate more than simple disappointment).

Stewart and Binoche are both good enough (Binoche less so: when the script is that specific about what's going on in the character's head, it makes it harder for the actor to do much discovery on her own) that the film is still worth seeing, though I concede that it's more an obligation than a pleasure at that point. And while Assayas's visual storytelling isn't here at the level it is in, say, Summer Hours, where camera movement is a function of character psychology and the visual representation of places trumps the screenplay in communicating the meanings of the film, he still shows some of the old talent for crafting leading images; the positioning of Valentine and Maria in the frame does better work of indicating the slithery conflict between the two women's psychologies more craftily and convincingly than anything in the lumpy script. Still, "there are reasons this isn't bad" is hardly a ringing endorsement, and for all its intermittent strengths, it's hard to think of Clouds of Sils Maria anything better than a compelling failure.