Double Agents, Dead Drops & Declassified Files: Inside the MI5 Archive of Official Secrets

For over a century, in a building most of us could not name, an organization known only by its post office box number — PO Box 500, London — has been quietly reading other people's letters, following other people's footsteps, and keeping the realm's most closely guarded secrets. MI5, the British Security Service, was founded in 1909 in the nervous run-up to the First World War under the motto Regnum Defende — Defend the Realm. James Bond and George Smiley are the fictions. This is the real thing. In this episode, Jonathan Thomas is joined by Mark Dunton, Principal Records Specialist at the National Archives and lead curator of their hit MI5: Official Secrets exhibition, and Dr. Stephen Twigge, former Head of Modern Collections at the National Archives and a historian of the Cold War and code breaking. Together they've written The MI5 Archive: Official Secrets, a remarkable visual record drawn from the 6,000 files MI5 has released since 1997. They explain what it actually feels like to hold these documents, how MI5 differs from MI6 and GCHQ, how a secret becomes an archive, and how historians work with a record that is deliberately incomplete. Then come the stories: the German spy hanged by a lemon, the last man executed at the Tower of London, the astonishing Double Cross system that invented 27 fictional agents to deceive Hitler about D-Day, the surveillance transcript that records atomic spy Klaus Fuchs stoking his own fire, the truth about MI5's limited role in the Profumo affair, and the handwritten note authorizing a tap on the King's telephone during the abdication crisis. Links The Book The MI5 Archive: Official Secrets — Thames & Hudson The MI5 Archive: Official Secrets — The National Archives Shop The MI5 Archive: Official Secrets — Thames & Hudson USA The MI5 Archive: Official Secrets — Waterstones The MI5 Archive: Official Secrets — Amazon UK The MI5 Archive: Official Secrets — Amazon USA Published by Thames & Hudson in association with The National Archives. 368 pages. Foreword by Nick Harkaway. ISBN 9780500030455. Our Guests Mark Dunton at The National Archives The National Archives, Kew Related Reading Double Cross: The True Story of the D-Day Spies by Ben Macintyre Agent Zigzag by Ben Macintyre Tinker Tailor Soldier Spy by John le Carré Places Mentioned The Tower of London — Historic Royal Palaces MI5 — Official Website GCHQ — Official Website Bletchley Park Anglotopia Friends of Anglotopia Club Related episode: Dr. Chris Smith on the Cambridge Five and The Last Cambridge Spy Takeaways MI5 has transferred over 6,000 files to the National Archives since 1997, and while most government departments operate under a twenty-year rule, MI5 works to an unofficial fifty-year rule. Strictly speaking they are not obliged to transfer anything at all — they choose to. MI5, MI6 and GCHQ are, as Stephen puts it, "brothers from the same mother." All emerged from the Secret Service Bureau created in 1909, which had a domestic side and a foreign side. The two heads shared an office for about a month before going their separate ways. MI5 catches other people's spies at home; MI6 runs Britain's spies abroad; GCHQ, which began as the Government Code and Cypher School under MI6, provides signals intelligence to both. Unlike the famously fractious CIA/FBI relationship in the US, the British agencies get on reasonably well — largely because they're staffed by lifetime civil servants rather than political appointees, and because people move between them (Dick White ran MI5 and then MI6). Less turf war, less budget anxiety. The exhibition MI5: Official Secrets — which ran April to November 2025 and drew over 55,000 visitors — was the first time MI5 has ever collaborated with another institution to tell its own history. The book contains a great deal that wouldn't fit in the exhibition space. Karl Muller, a First World War German agent, was caught because his invisible ink was lemon juice. Two lemons were found — one in his dressing table drawer wrapped in cotton wool, one in his overcoat pocket. He claimed they were for dental treatment. He was executed at the Tower of London. MI5 then kept writing letters in his name, asked the Germans for more money, received a handsome sum, and bought a staff car with it — which they nicknamed "the Muller." The Treasury told them off for unauthorized expenditure. The car was later stolen from outside MI5 headquarters. Josef Jakobs, parachuted into Huntingdonshire in January 1941, broke his ankle on landing and fired his revolver into the air to summon help. He was found with a German torch containing a German battery, a road map of Great Britain, a forged ID card, and a torn-up cipher wheel. Uncooperative under interrogation at Camp 020, he could not be turned — and became the last man ever executed at the Tower of London. The Double Cross System — officially the Twenty Committee, because XX in Roman numerals is twenty — turned virtually every German agent sent to Britain. MI5 then had to invent entire fictional networks of sub-agents to feed the Germans plausible material. Agent Garbo, a Spanish former chicken farmer who hated both chickens and Nazis, worked with his handler Thomas Harris to create around 27 wholly fictional sub-agents, all reporting on a non-existent army group massing in southeast England. Agent Tate convinced the Germans there was a large minefield south of Ireland. On 1 January 1945 British intelligence intercepted a message to all U-boat commanders warning them to avoid the area. That deception kept the Atlantic convoy route safe and saved countless lives. Tate also misled the Germans about where V1 and V2 rockets were landing, causing them to adjust trajectories so the missiles fell in open countryside rather than urban areas. On the Profumo affair, the files actually deflate the conspiracy theories. Contrary to the widespread impression that Stephen Ward was working for MI5 and was then abandoned by them, the files show MI5's involvement was genuinely limited — Ward met an MI5 officer two or three times, and was warned to be wary of Ivanov, not recruited. Director General Roger Hollis wrote that MI5 didn't get involved in that sort of "call girl racket." Stephen's most striking find is a handwritten note from the Prime Minister's office to the head of the GPO during the abdication crisis, authorizing the monitoring of King Edward VIII's communications at Fort Belvedere — alongside Special Branch maps tracking Wallis Simpson's daily movements. Nobody, it turns out, is above surveillance. Soundbites "You're never sure what's going to come next when you turn the page. You get a letter, and it can be quite mundane at times — how are the kids doing from so and so. And it takes you to places you don't necessarily go to in normal life." — Stephen on reading the files. "You get a very real sense that these were records which were not meant for public consumption. It's a bit like the title of the James Bond film — For Your Eyes Only." — Mark on handling MI5 documents. "MI5 and MI6 are, I suppose, brothers from the same mother. The two heads of those organizations shared an office for about a month and then went their own ways." — Stephen on the origins of British intelligence. "It's the first time that MI5 have ever collaborated with another institution to tell its history in this way." — Mark on the exhibition and the book. "There's this riveting transcript derived from a listening device placed in the home of Klaus Fuchs. And it records him moving about, with little details like 21:07, Fuchs stoked the fire. It gives you an almost voyeuristic feeling that you're looking in on this person's life and listening in with the MI5 listeners." — Mark on the atomic spy surveillance logs. "They asked him, what's this lemon for? And he claimed it was for dental treatment for his teeth. Which didn't necessarily go down well." — Stephen on Karl Muller's defence. "MI5 kept sending letters in his name, and in one of them asked for some money. They received quite a handsome amount, and then they bought a staff car with it, which was then called the Muller. Which, even more embarrassing, was stolen outside MI5 headquarters in the 1920s." — Stephen on the most delightful footnote in MI5's history. "He'd been a chicken farmer before the war, but apparently hated chickens. And he also hated the Nazis. And he had such a fertile imagination — together with his MI5 handler they cooked up about 27 fictional sub-agents." — Mark on Agent Garbo. "You get this bizarre wilderness of mirrors, where the German intelligence authorities think they're doing wonderfully well. They've got all these agents around the country feeding them information — but little do they know it's all bogus." — Stephen on the Double Cross system. "There's a letter that authorizes the tapping of the King's phone, which you wouldn't necessarily imagine to be released. It's a handwritten note from the Prime Minister's office. It's a fascinating idea that no one is above the law — everybody is capable of being monitored." — Stephen on the file that stayed with him. Chapters 00:21 Introduction — PO Box 500, Regnum Defende, and the real thing behind the fiction 01:17 Meet the Guests — Mark Dunton and Dr. Stephen Twigge of the National Archives 01:44 What Does It Feel Like to Hold These Files? 02:56 For Your Eyes Only — Mark on a window into a hidden world 04:13 How the Book and the Exhibition Came About — Approaching MI5, and their surprising response 06:11 The Exhibition — April to November, 55,000 visitors, and why the book holds more 07:04 Where MI5 Fits — MI5, MI6, GCHQ and the Joint Intelligence Committee explained 09:51 Do They Actually Get Along? — Comparing British and American intelligence culture 11:19 PO Box 500 — The anonymous address, and how a secret becomes an archive 12:16 What MI5 Has Been Called — Secret Service Bureau, MO5, Security Service, and Military Intelligence Section 5 13:12 Opening Up in the 1990s — Regular transfers and 6,000 files 14:14 Working with a Deliberately Incomplete Archive — Redactions, gaps, PF numbers and the jigsaw 16:11 The 50-Year Rule — Why MI5 files are often less redacted than other departments' 17:49 Outside the Normal Framework — They're not obliged to transfer anything at all 18:37 Do They Only Release What They Want Us to See? — Mark on the Cambridge Spies 19:04 What Makes a Good Exhibit? — Turning a dry surveillance report into something you can feel 20:06 Klaus Fuchs Stokes the Fire — The listening device transcript that lingers 21:33 THE FATAL LEMON — Karl Muller and the First World War 22:11 An Ill-Trained Agent — Latvia, Antwerp, and a very short course in secret inks 23:37 A.E. Free — Lemon juice, a compromised post box, and a German baker in Deptford 25:12 Caught in Russell Square — Lemons, German clothing, and a story about dental treatment 25:58 Execution at the Tower — And what MI5 did next 26:26 The Muller — Asking the Germans for money, buying a staff car, and getting told off by the Treasury 27:07 Two Lemons — Black, shrivelled, and in the collection 27:33 THE SIEGE OF SIDNEY STREET, 1911 27:55 Refugees, Radicals and the East End — The context for a gunfight 28:53 The Gun Battle — Scots Guards, machine guns, and a building burnt to the ground 30:57 Why It Matters — MI5's shift from military counter-espionage to civilian subversion 32:01 Enter Winston Churchill — The Home Secretary who turned up for the cameras 32:47 JOSEF JAKOBS — THE LAST MAN EXECUTED AT THE TOWER 33:03 A Broken Ankle in Huntingdonshire — And a revolver fired into the air 33:42 Camp 020 at Ham Common — Where the parachuted spies were interrogated 34:06 Who Was He? — A dentist, and an exploitative role in Jewish emigration 34:43 Uncooperative Under Interrogation — Why he couldn't be turned 35:38 Shot by Firing Squad — The last execution at the Tower of London 35:59 The Objects — A German torch with a German battery, a road map, a forged ID and a torn-up cipher wheel 36:40 THE DOUBLE CROSS SYSTEM 36:58 How It Worked — An offer they couldn't refuse, and inventing entire fictional networks 38:38 Chicken Feed and Burning Aerodromes — Making the deception plausible 39:15 A Wilderness of Mirrors — And Operation Fortitude 39:47 Why "Twenty Committee"? — XX in Roman numerals 40:28 Operation Fortitude in Detail — Distracting the Germans from Normandy toward the Pas de Calais 41:44 AGENT GARBO — A Spanish chicken farmer who hated chickens and hated Nazis 42:15 27 Fictional Sub-Agents — And a non-existent US Army group in southeast England 42:51 Agent Treasure and Agent Tate 43:26 The Minefield South of Ireland — The U-boat warning of 1 January 1945 44:32 Saving the Convoy Route — And countless lives 44:36 The V1 and V2 Deception — Redirecting missiles into open countryside 45:04 What Happened to Them Afterwards? — Garbo's shop in South America, an MBE, and Ben Macintyre's research 46:18 THE COLD WAR — How MI5's job changed after 1945 46:38 Atomic Spies at Harwell — And higher stakes after 1949 47:20 From War Footing to Peacetime — Reduced budgets and a new focus on subversion 48:18 The Communist Party of Great Britain — An inordinate amount of attention 48:30 A Shadow War — Intelligence as back channel, and why sex scandals made the front pages 50:05 THE PROFUMO AFFAIR, 1963 50:25 The Cast — John Profumo, Christine Keeler, Stephen Ward and Eugene Ivanov 51:40 What the Files Actually Show — MI5's involvement was limited 52:36 Stephen Ward and the Man Called Woods — Warned about Ivanov, not recruited 53:15 Ward Reported Back Conscientiously — And was not working for MI5 53:52 Roger Hollis on the "Call Girl Racket" — And what the files do to the conspiracy theories 54:54 FICTION AND REALITY — Nick Harkaway's foreword, le Carré and Fleming 55:18 What le Carré Got Right — Moles, lamplighters, honey traps, and a vocabulary that entered common usage 55:53 It's Not James Bond — It's people sitting there watching people and writing down what they did 57:09 The Doris Lessing File — Why she was on MI5's radar, and how rounded these portraits are 57:46 Fleming and Naval Intelligence — Special Operations Executive and the daring-do end of the business 58:29 THE FILE THAT STAYED WITH THEM 58:42 Stephen's Choice: The Abdication Crisis — Tapping the King's phone at Fort Belvedere 59:28 Special Branch and Wallis Simpson — Maps of her daily routine 59:56 Mark's Choice: The Lemon — And a "license to peel" 01:01:04 All the Little Twists and Turns — Why the Karl Muller story is the perfect MI5 story 01:01:34 Wrap-Up — The book, publication details, and Friends of Anglotopia Video Version