British espionage historian Christopher Andrew joins MI5 to examine their historical archives as part of his research on the Service's 100 years of existence.
| Actor | Role |
|---|---|
 Matthew MacfadyenSelf - Narrator (voice) | Self - Narrator (voice) |
| CChristopher AndrewSelf - Author 'Defence of the Realm' | Self - Author 'Defence of the Realm' |
| SStephen LanderSelf - MI5 Director General 1996-2002 | Self - MI5 Director General 1996-2002 |
| CCynthia PostanSelf | Self |
 Harold WilsonNot listed | Not listed |