Struggling to connect with those around him, a teenage math prodigy trains for the International Mathematical Olympiad, where a budding romance with a fellow contestant breaks through his emotional walls and teaches him that love cannot be calculated.
| Actor | Role |
|---|---|
 Asa ButterfieldNathan Ellis | Nathan Ellis |
 Rafe SpallMartin Humphreys | Martin Humphreys |
 Sally HawkinsJulie Ellis | Julie Ellis |
 Eddie MarsanRichard | Richard |
 Jo YangZhang Mei | Zhang Mei |
 Alex LawtherIsaac Cooper | Isaac Cooper |
 Alexa DaviesRebecca Dunn | Rebecca Dunn |
 Martin McCannMichael Ellis | Michael Ellis |
 Orion LeeDeng Laoshi | Deng Laoshi |
 Jake DaviesLuke Shelton | Luke Shelton |
 Paul J. DoveJohn | John |
 Percelle AscottBen Morgan | Ben Morgan |
| CChristina LowIMO Contestant | IMO Contestant |
| EEdward Baker-CloseNathan Ellis (9 yrs) | Nathan Ellis (9 yrs) |
| SSuraj RattuPav Kamdar | Pav Kamdar |
 Jamie BallardHeadmaster | Headmaster |
 Clare BurtDoctor | Doctor |
| AAdam FosterPaediatrician | Paediatrician |
| LLee Zhuo ZhaoLi Jing | Li Jing |
 Shannon BeerCaz | Caz |
 Tasha ConnorJessie | Jessie |
| BBo-Han HuangLuke's Buddy | Luke's Buddy |
| SSong-Hung ChangEmployee | Employee |
| LLawrence JeffriesSchoolboy | Schoolboy |
| CCiaran WakefieldSchoolboy | Schoolboy |