Una is like most stage-to-screen adaptations. Which is to say that it’s more impressive at evoking the vestiges of what would have made it a successful play without replicating them. While plenty of plays have made the transition to film unscathed or even improved upon (Who’s Afraid of Virginia Woolf?, Amadeus, Glengarry Glen Ross), too many are pale imitations of their on-stage electricity.

Now, to be fair, I haven’t seen the play Blackbird, as written by David Harrower (who reinterprets it for cinema here). But it’s easy to see how effective it would have been properly staged as a claustrophobic revisitation of a traumatic past that twists into something darker and more complex. This is a story of childhood sexual abuse, a story of a survivor, Una (Rooney Mara) and a predator, Ray (Ben Mendelsohn). Now in her twenties, Una confronts Ray at his place of work and the confrontation opens up new wounds while complicating – in controversial fashion – the convention narrative of abuse.

On stage, I have no doubt the pacing – the escalation, the revelations, the ramping intensity – of this story would have been captivating. I believe that because it’s evident in Una’s final act, in which psychological walls crumble in devastating fashion. Before that, though, the film stumbles through its unquestioning embrace of cinematic technique. I’m referring to two techniques in particular.

Firstly, there’s director Benedict Andrews’ mise-en-scéne. Presumably anxious to accentuate the film’s emotional journey from bitterness and distance to perverse intimacy, many of the early scenes between Ray and Una are staged with an alienating distance. Sometimes literally – with Andrews shooting warehouse conversations between the pair from afar – and sometimes through the emphasis of the industrial workplace accoutrements around them. In theory, it’s a clever choice, but it’s so overtly executed to create too much distance between the audience and these characters.

That’s exacerbated by the film’s extensive use of flashbacks. This is an understandable choice but, again, a mistake. By allowing our knowledge of these events to be shaped entirely by Una and Ray’s recollections, it denatures any idea of a ‘true’ recounting while ratcheting up the emotional intensity and our empathy with these characters. These flashbacks would’ve been impossible to execute on stage, of course, but just because they’re possible on-screen doesn’t mean they elevate the story.

These are understandable missteps. Andrews, whose previous directing experience was solely in the theatre, is looking for opportunity to distinguish this adaptation from its source. While I’ve griped – and will continue to grip – about unimaginative interpretations of plays that fail to exploit the potential of cinema, I think Una would’ve been a stronger film had it kept a tighter focus.

Such complaints aside, the film is far from a failure. When it ramps up into the third act, which unravels the psychology of Una and Ray, Una is consistently gripping and provocative. Freed of the awkwardness that plagued earlier scenes, Mara and Mendelsohn take ownership of the material. Mendelsohn, in particular, resists the apparent ambiguity of the screenplay: he understands who Ray is, what he wants, and he’s resolute in conveying that on screen.

Still, for all Una’s strengths, its primary achievement is making you really want to seek out a staging of the original play.