The tyranny of awards-season chatter is that it reduces interesting movies to uninteresting horse race conversation, so let's get the dull part out of the way: yes, Brie Larson is fantastic in Room. This is only surprising if you haven't been paying attention. Brie Larson is always fantastic. She was fantastic in Don Jon, where she played an almost dialogue-free role built around the most obvious possible gag. Obviously she'll be fantastic when handed a gift-wrapped role like this one. She has been more fantastic in other things - the one time I've seen Room and her in it will do for me, whereas her work in Short Term 12 is something I like to linger on and return to. But if she wins her first of what I pray will be multiple Oscars for Room, no cosmic injustice has been done.

And what of the movie itself? Honestly, reader, I consider myself somewhat unfussed, and that is maybe the worst possible response to a film with such an intensely bleak scenario. Several years ago, as a teenager, a woman whose name is withheld long enough that it's worth ignoring the fact she has one - this is being Larson's role - was kidnapped and thrown in a one-room shed by a man she calls Old Nick (Sean Bridgers). Here he has raped her on a regular basis, impregnating her, and that is how she has come to have a companion in that little room, five-year-old Jack (Jacob Tremblay). Jack will be our identification character for the entire run of the feature, which makes things somehow more ineffably awful: knowing what a vivid hell their crappy little home is and what a psychopathic beast Old Nick truly is is intensified by having all of it transmuted through the guilelessly whimsy of a little boy.

"Whimsey", perversely, is exactly the right word. Jack and Ma have created a robust ecosystem of imagination, personalising everything with nouns functioning as names - Room is the place they live, along with Chair and Toilet and Door and everything else. This is astonishingly gripping, shot through with warped absurdist comedy, a cockeyed fairy tale that Larson's performance darkens with her exceedingly tricky way of playing a sweet optimist for Jack (and, presumably, Ma herself), while being fully aware, in every moment, of her suffering. Credit, too, to screenwriter Emma Donoghue (adapting her own novel), for providing the erratic storybook dialogue that gives this all such a twisted edge, and to director Lenny Abrahamson and cinematographer Danny Cohen, for shooting the interiors with enough angles to give Room a weird vitality all its own.

There comes a point - I won't say when or how, other than to suggest that the bridging sequence is top-flight thriller filmmaking, and Tremblay is astonishing in it - when the captors return to the life Ma left those many years before, and Room shifts into a story of PTSD, of re-building a sense of self after even a very fucking awful form of stability has been removed, and of the difficulties of being around a family that tries and fails to understand oneself. In principle, this is exactly what the film should do. In practice, Room takes a hit here from which it never recovers. The script goes through a great deal of dead wood on its way into the second half, and there's a specific piece of casting that almost kills the whole second half of the movie outright. William H. Macy, an actor whom it is impossible to see without carrying certain expectations and baggage, unless this is the very first indie movie you've ever seen, gets a spear-carrying role as Ma's tense and horrified father who can't deal with thinking about what happened to her, and the lack of proportion between star and role size is distracting in the worst ways. Moreover, his character is set up with really leading dialogue and really telling shots as an alcoholic. The movie totally abandons this plot thread and character with no fanfare at all, leaving a grarled skein of dangling plot thread that never stop getting in the way of the rest of the movie.

Joan Allen, as Ma's own mom, is worked into the movie far more elegantly (giving a lovely, compact performance in her own right, impressively filtering herself twice: it's Jack's impression of Ma's impression of grandma's character that Allen is playing), and once it's clear that Macy and his subplot aren't coming back, and once it's clear that Jack's arc is over and we're now seeing Ma's play out secondhand for the rest of the movie, Room starts to settle into something meaningful again. But there's no excuse for it to be so sloppy in getting to that point, and something very special and important dies out of the movie. Larson tries hard, and she sells a great character, but the movie is at a loss for what to do with that character for a cool 30 minutes; they're 30 minutes that suck all the wind out of what should have been an all-time harrowing character study.