At first glance, Sebastián Silva’s Nasty Baby appears to slot neatly into the Noah Baumbach/Lena Dunham school of New York indie. You know the type. Loose, naturalistic dramas about trendy people in a trendy city, leavened with a hint of comedy before drifting towards Serious Issues that aren’t taken all that seriously. For most of its first half, it holds pretty closely to the formula – if that’s the word – before a dramatic third act twist derails it from its sub-genre path altogether. Without delving into specifics, this isn’t a “Baumbach” kind of ending.

This change in direction is invariably described as “divisive” and, for once, that adjective isn’t merely a euphemism for “bad” – audience reaction seems to be equally split between “interesting” and “disastrous.” Upon reflection, Nasty Baby’s third act seems to be thematically – if not dramatically – necessary, a decision that enriches and complicates themes of prejudice, fairness and empathy that run through the film. It may not be the lightweight indie drama promised on the packaging, but since when is subverting genre conventions a bad thing?

(Fair warning: I’m going to do my best to tiptoe around plot particulars, but astute readers will probably be able to feel out the shape of the aforementioned twist, so if you’re especially spoiler averse you might want to wait until you’ve seen the film yourself.)

Nasty Baby’s three leading players are pretty well in line with the kind of protagonists you’d expect from a New York indie. There’s Freddy (Silva), an up-and-coming artist moving from visual art to video art with a concept piece called – you guessed it – “Nasty Baby”; his partner, Mo (TV on the Radio’s Tunde Adebimpe); and their mutual friend Polly (Kristen Wiig), with whom they plan to have a baby. Together they form a tight-knit trio; less a couple-plus-surrogate, more an eclectic, unconventional family.

A less ambitious film might be content to spin out drama from the tensions and complications that eventuate in the wake of their struggle to bring life to the world – Mo’s tight-lipped reticence, Polly’s ticking biological clock, Freddy’s sparse sperm count. Instead, the three are drawn together more tightly by the slings and arrows of the outside world, with variations of xenophobia and homophobia directed their way. Mo’s family greets his relationship with politely devastating scepticism, but the major destabilising influence is a belligerent neighbour, known only as “The Bishop” (Reg E. Cathey). The Bishop’s harassment is initially innocuous – disturbing their morning sleep with a leaf blower, hassling them to purchase junk from his stoop – but soon things take a sinister turn, as he harasses Polly as she walks home at night and yells homophobic epithets at Freddy.

The Bishop ends up being at the epicentre of that aforementioned twist … though maybe ‘twist’ is the wrong word. The ensuing events are shocking in their violence, sure, but they feel thematically inevitable, the consequences of a pre-destined conclusion. The third act is challenging, but not so much because of said violence – granted, that might have provoked negative reactions from unprepared patrons – but in how it challenges audience identification. Suddenly these protagonists who we identify with – because they’re the subjects, sure, but also because they’re minorities, they’re gay, they’re victimised and marginalised – are revealed to be deeply morally compromised. Unlike so many films that go out of their way to avoid portraying such unconventional families as perfect, Nasty Baby presents them as real people – ultimately ugly, selfish, flawed people – but real people whose flaws have nothing to do with their sexuality (or, in the case of Polly, their lifestyle choices).

Nasty Baby also (indirectly) provokes its audience to consider the distinction between Freddy/Mo/Polly and The Bishop. While Freddy/Mo are subject to homophobia throughout, they possess substantial privilege when compared with their derelict neighbour, who’s clearly mentally ill, borderline-destitute and trying to support a family. The Bishop is a victim of the system; his homophobia an understandable side effect of an unequal society. Nasty Baby demands that we empathise – not sympathise – with this man and his hatred, and refuses to allow us to categorise individuals into ‘good’ and ‘bad’ baskets based on our politics.

That’s not easy to accept. Nor is it easy to embrace a film that presents itself as one film then pivots into another, tearing off its glossy veneer in the process. Nasty Baby is certainly not a perfect film (for starters, Silva is a better director than actor, while Adebimpe’s character is underused and underdeveloped), but perhaps those bemoaning its “poorly conceived” final act should stick to genre films that avoid such divergences from formula, and avoiding challenging the audience’s moral compass. Maybe find out if there’s a film called Nice Baby out there somewhere.