X+Y

Nathan is nine and has just lost his father in a car accident. The tragedy coincides with an autism diagnosis that explains his guarded, one-on-one way of relating to others. He relies on his mother Julie, who becomes his caregiver and scaffolds his world around math and routine.

To keep him focused on his studies, Julie enrolls him in advanced maths classes at High Storrs School in Sheffield. Nathan's social anxieties persist, and he clings to the mentor–student dynamic that helps him navigate new surroundings.

Martin Humphreys, a maths genius with multiple sclerosis, recognizes Nathan's potential and becomes his mentor. He guides Nathan to apply his prodigious talent toward high-level competitions while managing the realities of illness.

Seven years pass and Nathan remains focused on a path to the International Mathematical Olympiad. Martin continues to mentor him as they schedule a two-week maths camp in Taiwan to study for the test.

Nathan earns the chance to accompany 15 other British teenagers to Taiwan for intensive training. He fears he may not qualify, but his sustained performance wins him a place on the team.

In Taiwan, Nathan confronts social anxieties and discovers he is not the only prodigy in the room. He bonds with Zhang Mei, a Chinese classmate who helps him adapt and face his anxieties.

Back in England, Nathan and Zhang barely make the cut to compete in Cambridge, with Zhang staying close to the team as they prepare. Her presence gives Nathan a new confidence and shifts the dynamics of the group.

Zhang stays with Nathan and Julie at home, and her influence begins to transform Nathan's behavior into something more normal. Julie wonders if Nathan is developing feelings for Zhang and wrestles with what that means.

The trio travels to Cambridge for the Olympiad, settling in for the competition and continuing their study and preparation. Zhang's influence continues to shape Nathan's experience in unexpected, hopeful ways.

Zhang's uncle discovers her in Nathan's room one morning, prompting her withdrawal from the competition and return home. Nathan feels torn between his growing feelings and the Olympiad that remains his goal.

During the opening question of the Olympiad, memories of his late father surge to the surface and threaten to overwhelm him. The emotional overload drives Nathan to rush out of the exam hall, seeking air and relief.

In a quiet cafe, Martin and Julie listen as Nathan talks about his grief and the girl who understood him. They share a hopeful moment, and Julie drives him to the station to fetch Zhang Mei, signaling a return to balance on his remarkable journey.