Back in 2007, veteran British documentarian Morgan Matthews made a film called Beautiful Young Minds about teen mathletes vying to compete at the annual International Mathematical Olympiad for high-school geniuses. He was so inspired by them that he was moved to tell a fictionalized version of their trials and triumphs, and so here we have X+Y, his first narrative film. Nathan (Asa Butterfield: Ender’s Game) isn’t like the typical hero of a tale of adolescent angst: he’s autistic, and he’s much better with numbers than with people. But he blossoms when his single mother, Julie (Sally Hawkins: Paddington) — who is desperate to find some way to connect with her son emotionally and to give him an intellectual challenge that he isn’t getting at school — hooks him up with cranky math tutor Mr. Humphreys (Rafe Spall: Get Santa), which sets Nathan on the road to the IMO. At the training camp in Taiwan, Nathan meets another IMO hopeful, Zhang Mei (Jo Yang), and unlike in other similar stories of picking one’s way through the minefield that is teenagerdom, it’s far from certain whether romance will blossom, given Nathan’s particular difficulties. This is an honest, heartfelt film, full of lovely performances; Hawkins is a marvel, as always, but Butterfield is the real revelation here: he is absolutely heartbreaking as a young man seemingly at war with his own emotions. Still, the film is more effective earlier on, before it falls into a downward spiral of clichés and sentiment that is at odds with where the story seemed to be heading at first. “I’m usually the weird one,” Nathan marvels once he is among fellow weirdo math nerds and suddenly feels “normal” by comparison. His story, too, ends up rather unexpectedly conventional.