Rating: ★★★☆☆

The value of organisation cannot be under-appreciated in the world of filmmaking, for though the appeal of improvisation is to reveal something more honest and authentic, in the wrong hands, it can be a tightrope walk at best. Take Lynn Shelton’s (Humpday) latest, Your Sister’s Sister, a ripe, superbly-acted drama which very nearly completely falls apart in its underwritten third act.

Jack (Mark Duplass) is in dire emotional straits following the sudden death of his brother. Longtime friend Iris (Emily Blunt) stages an intervention of sorts, suggesting he visit her father’s log cabin for some solitude and time to think. Upon arriving, he discovers that Iris’ sister, Hannah (Rosemarie DeWitt), is there, herself taking time away to ponder a long-term relationship she has just abandoned. After an awkward, drunken sexual encounter, the two awaken to find Iris making a surprise visit, while they desperately try to keep their tryst under wraps.

Like most mumblecore films, the Devil is not so much in the detail of the plot as it is the emotional nuances of its characters. For most of Shelton’s film, this is a grand strength, emphasising Jack’s fragile state, while his and Hannah’s mutual anguish allows them to find in each other a small measure of comfort over the course of a Tequila-filled evening. The arrival of Iris – and along with her, a love-and-other-things triangle – may feel like a more conventional source for cringey comedy, but Shelton and her three leads milk this excruciating tension for all it is worth throughout the brief 90-minute runtime.

As with the best films of its kind, Your Sister’s Sister is constructed and performed with a passion that gives an at once relaxed and authentic feel. There is no rush here to pulse through plot beats – for the set-up alone comprises almost half of the film – and instead Shelton slowly gathers the dramatic stakes into a pile before letting loose with the inevitable explosion.

And for 65-or-so-minutes of its running time, Shelton holds a firm grip of both her characters and natural trajectory of her story. A rather melodramatic bombshell dropped late in the day, while promising in terms of the dilemma it presents, writes Shelton and her ad-libbing performers into a taxing corner, one which they are unable to extricate themselves from in the remaining 20 minutes of screen time.

One avenue to take when presenting the viewer with a punishing choice is simply not to make it, and leave the answer in the hands of the audience to dissect and discuss forever more. That Shelton instead feels obliged to move the characters out of their angry, stewed state into one of forgiveness and acceptance gives those final scenes a disingenuous feel. Thus, the cliffhanger on which everything ends – a fine one in of itself – is propelled by acts and emotions many will find it impossible to connect with. Talking over their problems some more in the wake of “the twist” might have remedied this, but any which way, it is an incredibly hard sell.

Things are so right for so long that its third-act departure is especially disappointing, almost derailing the whole thing, if not for the three charming and committed performances. While Blunt and DeWitt are unsurprisingly stellar, it is Duplass, typically working behind the camera with brother Jay, who disarms and impresses here, as an emotionally complex, put-upon character. Hopefully this is something he will continue to nurture alongside his directorial work, and given this film’s key flaw lies in the writing, the thought of him starring in more of his own films is something to cross your fingers for.

Like so many well-intended dramas, it stumbles at the tallest of hurdles – fashioning emotional credibility. However, there is so much good in the first hour that you can just about forgive its poorly written third act.

Your Sister’s Sister is in cinemas now.

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