What sounds like a certifiable snooze-fest (a socially awkward teenager hopes to win an international maths competition), turns out to be an unexpectedly touching drama.

A little because a uniformly fine cast hit all the right notes, a lot because the story itself is unexpectedly touching. It's all the more surprising when you consider writer James Graham has only written a couple of TV episodes while director Morgan Matthews has, until now, worked exclusively in documentaries. Yet together they've tapped a nerve that speaks to the heart of every kid (and the worried parent of such a kid) who felt like a complete outsider until that day their world changed forever.

For Nathan (Hugo's Asa Butterfield), that was the day Humphreys (Rafe Spall - Life Of Pi) became his tutor. Nathan sits somewhere on the spectrum, a boy who finds everything in life difficult except for mathematics. He's so good at it, his widowed mother (Sally Hawkins - Blue Jasmine) hires the one-time maths genius to help her son to the International Mathematics Olympiad. The pair soon connect, a little through the wonder of maths but also through disability; Humphreys has a physical disorder that dropped him outside life's comfort zone and by chance, into Nathan's.

Here is a film that manages to be challenging, endearing, sweet and romantic all without breaking a sweat. Credible performances are the core of this achievement, with the always excellent Eddie Marsan (Still Life) as a fellow tutor key to that support. In putting an honest face to spectrum disorder and the simple horrors of teenage anxiety, Matthews avoids stereotypes and sentimentality to create tangible people coping with the pressures of the world around them. A sub-plot involving a victim of bullying in what should be a safe space is particularly effective.

X+Y is not without a couple of clunky transitions that don't bear close inspection, but they're no significant distraction from what is a thoughtful, moving and often funny reflection on those most troubling years.

// COLIN FRASER

Previewed at Sony Theatre, Sydney, on 20 January 2015