- Steven Mackintosh
- Self - Narrator
- Brian Deacon
- Gordon Welchman
- Rosamund Welchman
- Self - Gordon Welchman's Daughter
- W. Gordon Welchman
- Self - Ex-Head of Hut 6, Bletchley Park (1939-1941)
- Sinclair McKay
- Self - Author, 'The Secret Life of Bletchley Park'
- Jimmy Thirsk
- Self - Veteran, Hut 6
- Asa Briggs
- Self - Veteran, Hut 6
- Jane Fawcett
- Self - Veteran, Hut 6
- Tony Comer
- Self - GCHQ Historian and Former Analyst
- John Naughton
- Self - Vice President, Wolfson College, Cambridge
- John Scarlett
- Self - Chairman, Bletchley Park Trust, and Former Chief, MI6
- Jennifer Welchman
- Self - Gordon Welchman's Granddaughter
- Robert R. Everett
- Self - Former President, The MITRE Corporation
- Eric Ellingson
- Self - Gordon Welchman's Colleague
- Larry Hill
- Self - Gordon Welchman's Colleague
- Rupert Allason
- Self - Security and Intelligence Historian
- Diana Lucy
- Self - Welchman Family Friend
- Skye Duncan Wentworth
- Self - Gordon Welchman's Neighbour
- Dave Hatch
- Self - Official NSA Historian
- John C. Stevens
- Self - Welchman Family Lawyer
- James Bamford
- Self - Author, 'The Puzzle Palace'
- Cynthia Storer
- Self - Former CIA Analyst
