Greta Gerwig and Lola Kirke just about save Noah Baumbach's hyperventilating comedy-drama from ruin

Betrayal is one of the first words mentioned in Mistress America. With a penny-dropping sound, you realise it’s writer-director Noah Baumbach’s pet theme. It’s the feeling produced by a bitter divorce in The Squid and the Whale, the sisterly combat in Margot at the Wedding, the inter-generational turf wars in While We’re Young. Friendships, especially, are vulnerable creatures in Baumbach’s work: they slide away or combust in a flame of acrimony.

This makes the newly forged one in Mistress America particularly tense to watch. It’s oddly un-Baumbach-like to announce his subject so openly and then draw up a plot which just sets it in motion, like a ticking time-bomb. He can be an evasive, slippery filmmaker: here he’s confronting something head-on, but somehow seems more self-conscious than ever. What makes Mistress America peculiarly frustrating, though, is what great potential it whips up – for a good half-hour it’s a fast and fluid pleasure, waiting to curdle.

First, we meet Tracy (Lola Kirke), a college freshman who dislikes her peers and is disliked back. She wants to write, but has no experience from which to do so, which is possibly why her first submission to a literary magazine gets rejected. Her second attempt, which we spend most of the film waiting to hear about, is “inspired” by an encounter with Brooke (Greta Gerwig), a 30-year-old Manhattanite she’s just met, who’s about to become her step-sister.

Something sparks instantly between these two. It’s no ordinary kinship – they talk at cross-purposes, inserting discordant non sequiturs between each other’s lines. Baumbach spikes his editing rhythms to suggest a headlong mutual appreciation, so giddy it’s almost trippy, but there’s also, from the word go, a competitive edge to it.

Brooke would rather deny there’s an age gap between her and Tracy, because she’s spent those years doing little but dawdle, letting bright ideas burst above her head like light bulbs with wonky filaments. (Her latest, patently doomed plan is to open a bodega-like restaurant “where you also cut hair”.) She’s broke, reliant on someone abroad she calls her boyfriend, who dumps her and gets her locked out when he sees an online photo of her kissing someone else.

Brooke’s disaster-prone air is attractive, both to Tracy and us, because of the sheer charisma of Gerwig, who adapts her flailing persona from Frances Ha into something more strenuous – she’s bossier, and pretends to be surer of herself, but there’s no floor under Brooke to support all these projects she’s dreaming up. You can see why Tracy wants to make her the tragicomic heroine of her newest literary effort, but she holds on to this as a dirty little secret, and the film starts to buckle, at the halfway point, from the weight of carrying it around.

The problems multiply when Brooke, Tracy, and two of her nervier contemporaries pay a surprise house-call upstate on Brooke’s super-rich ex-boyfriend (Michael Chernus), interrupting a book group being hosted by his wife (Heather Lind). Baumbach settles in here for a long, shouty, single-location grudge match with eight characters firing off exhausting barbs and aperçus at each other.

Earlier, ironically, his script contains a disobliging mention of Other Desert Cities, a 2011 Broadway play by Jon Robin Baitz – but we wind up in exactly that mode of hyperventilating comedy-drama, and we'd be better off out of it.

What fends off ruin are the central performances. In Kirke, who played a shifty opportunist in Gone Girl, Baumbach has not only cottoned onto an invaluable partner for Gerwig, but also an actress who succeeds in being interesting by herself, with her mush-mouthed delivery and touchingly naïve emotional confusion. The film’s far more persuasive at suggesting what Lola Kirke has to offer, and what Gerwig’s still got, than doing their characters, or Baumbach, any favours.