 Benedict CumberbatchAlan Turing | Alan Turing |
 Keira KnightleyJoan Clarke | Joan Clarke |
 Matthew GoodeHugh Alexander | Hugh Alexander |
 Rory KinnearDetective Robert Nock | Detective Robert Nock |
 Allen LeechJohn Cairncross | John Cairncross |
 Matthew BeardPeter Hilton | Peter Hilton |
 Charles DanceCommander Denniston | Commander Denniston |
 Mark StrongStewart Menzies | Stewart Menzies |
 James NorthcoteJack Good | Jack Good |
 Tom Goodman-HillSergeant Staehl | Sergeant Staehl |
 Steven WaddingtonSuperintendent Smith | Superintendent Smith |
| IIlan GoodmanKeith Furman | Keith Furman |
 Jack TarltonCharles Richards | Charles Richards |
 Alex LawtherYoung Alan Turing | Young Alan Turing |
 Jack BannonChristopher Morcom | Christopher Morcom |
 Tuppence MiddletonHelen | Helen |
 Dominic CharmanSherborne Student 1 | Sherborne Student 1 |
 James G. NunnSherborne Student 2 | Sherborne Student 2 |
| CCharlie MantonSherborne Student 3 | Sherborne Student 3 |
 David CharkhamJoan's Father | Joan's Father |
 Victoria WicksJoan's Mother | Joan's Mother |
 Andrew HavillTeacher | Teacher |
| LLaurence KennedyHeadmaster | Headmaster |
 Tim van EykenMI6 Agent | MI6 Agent |
 Will BowdenMilitary Policeman | Military Policeman |
| MMiranda BellMargaret | Margaret |
 Tim SteedElectrical Assistant | Electrical Assistant |
| HHayley Joanne BaconWoman in Crowd (uncredited) | Woman in Crowd (uncredited) |
 Nicholas BlattMinistry Man (uncredited) | Ministry Man (uncredited) |
 Ancuta BreabanWren (uncredited) | Wren (uncredited) |
