Proof, Menier Chocolate Factory, London

Proof tells the story of Robert (Matthew Marsh), a one-time maths genius who, now middle-aged, suffers from an unspecified, sporadic madness and general mental decline, after his skills peaked at the age of 25. It focuses on his daughter, Catherine, played by Mariah Gale, who is also very gifted at maths, but gave up a place at college to care for him full-time. The set is the back garden of their dilapidated home in Chicago, all crumbling wood and decaying plants.

There are plentiful math-geek jokes, often told with the distant, autistic gaze of those good with numbers but bad with people. But the tale is essentially a sad one with occasional moments of lightness.

Gale uses her impressive ability to cry at will by spending most of the time either in tears or on the verge of tears, which, though moving, offers little variety. We are well into the second half before she smiles for the first time, just after she gets laid. But her sister Claire’s overbearing fussiness, played with convincing concern by Emma Cunliffe, provides an excellent counterpoint to Gale’s dry wit and depressive outlook.

For a play whose central plot device involves complicated maths, there’s very little pretence at making the mathematics sound convincing - we never learn how these complex equations scribbled in notebooks might apply to the real world, so there’s a constant reminder that the maths is made up. Pair that with the imperfect Chicago accents and the viewer can never fully immerse themselves in the world of the play.

The script leaves a lot of holes that the direction (by Polly Findlay) doesn’t attempt to fill. After initially asserting that Catherine is suffering from the same vague malady as her father, it brushes it under the carpet. It might have been more interesting to toy with this area of doubt. As the play’s title suggests, there are several grey areas that the director, like the characters, seemed to want to sew up neatly and prove one way or another - is Catherine’s love interest out to exploit her to get to her father’s work?

Is the entire play a figment of her imagination? Did she try to pass off her father’s work as her own? It’s in these areas of intrigue where the play could be lifted from a story with several plot gaps to one that could keep you guessing at the ambiguities. Instead, a need for proof and certainty rules, leaving little room for doubt.