In his own words, trail author Gavin Greenwood 'has spent some time on the fringes of the secret world of spies and spying.' Greenwood has constructed a fictional narrative to explore the workings of wartime espionage.
In the text you will find italicised links to pages with more detailed information about the story, as well as museums and locations you can visit and websites of interest.
In late 1941 a Norwegian resistance Military Intelligence (XU) group operating around Trondheim fjord identify an unfamiliar submarine leaving a German naval base. A coded message is quickly sent to Britain's Secret Intelligence Service (SIS - later to become MI6) via neutral Sweden.
The SIS passes the message on to the navy. In the absence of any other confirmation, a senior naval intelligence officer 'informally' contacts a colleague working with the 'Ultra' codebreakers at Bletchley Park to check if they have any further information. The alert allows them to focus on German naval cipher traffic in the Norwegian area.
At Bletchley Park an army of codebreakers work round the clock to crack codes. Picture courtesy Bletchley Park.
To find out more about the story of Enigma and the secret work at Bletchley Park click on this link.
Bletchley's analysts find evidence that a German submarine (U-boat) has recently been ordered from Norway to a position in the North Atlantic. U-boat detection became a key aim of the physical war effort and the intelligence war. The deciphered signals indicate a large German U-boat 'wolfpack' is forming close to key convoy routes between North America and Britain.
This vital intelligence is passed on to the Joint Intelligence Committee (JIC) in London, who in turn inform the War Cabinet. Today visitors can enter the top secret complex where the War Cabinet met, restored to the way it was during wartime.
Deep beneath the streets of Whitehall was the Cabinet War Room, a fortified basement where the cabinet could meet without fear of falling bombs or prying eyes. Photo courtesy Imperial War Museum.
Learn more about Churchill's secret war by visiting the Cabinet War Rooms - click on this link to find out more.
After consideration at cabinet level, the navy and air force are alerted over the U-boat sighting, while the War Cabinet calls on the SIS and the Special Operation Executive (SOE) to gather more information on the mysterious submarine - using any means at their disposal.
The Special Operations Executive had many hidden bases in central London where agents were interviewed, trained and dispatched to their areas of operation. Click on this link to find out more.
A few weeks later SOE receives a report that one of its agents at L'Orient in western France has photographed what she describes as an 'unusual' U-boat entering the German submarine base in the port.
SOE heroine - Violette Szabo was one of hundreds of SOE agents who undertook dangerous missions into occupied Europe. Her life ended tragically at the hands of the Gestapo. Picture courtesy Violette Szabo GC Museum.
To find out more about the life and tragic death of Violette Szabo and the museum dedicated to her life click on this link.
A decision is made to retrieve the covert pictures. Because of a high degree of German military activity in the region and doubts over the loyalty of some of the local population, the photographs are taken by courier to a farm near Alencon in southern Normandy.
On a moonless night a few days later, a RAF Lysander aircraft operating from RAF Tangmere (West Sussex) is guided in to land by local resistance fighters. The aircraft delivers two agents and some equipment, collects the pictures and returns to Britain.
When flying into enemy occupied Europe, agents were often equipped with a vast array of bizarre apparatus. Click on this link to find out more.
With its long range and two seat cockpit, the Westland Lysander soon became the preferred mode of transport for SOE agents in their top secret forays in and out of Nazi occupied Europe. Painting by Douglas Littlejohn. Photo courtesy: Dr J Tanner.
Many wartime secret missions were flown from UK airfields deep into enemy territory delivering supplies and agents. Two of these bases can be visited today - click on this link to find out more.
Once back in the hands of British Intelligence, the photographs are studied by naval intelligence, who quickly realise that the U-boat has been adapted to supply other submarines with fuel, torpedoes and other provisions, thereby extending their patrol duration and range.
This development causes alarm because any increase in the time U-boats can spend at sea, where they are least vulnerable to attack, would add to the problems faced by already over-stretched naval escorts seeking to protect merchant shipping convoys.
The government makes detection and destruction of these new types of U-boat a priority. Bletchley and other intelligence organisations focus their resources on finding the submarines, while the navy and air force are ordered to always attack them regardless of the circumstances.
Every year brings a new batch of top secret wartime documents into the public domain. These can be accessed at the Public Record Office. Click on this link to find out more.
Careless Talk Costs Lives - a British wartime propaganda poster held by the Public Record Office. Picture courtesy Public Record Office.
This scenario was rarely so smoothly implemented in reality. Suspicion and hostility within resistance cells in occupied Europe, inter-departmental rivalries and feuds among the intelligence agencies in Britain, German counter-measures and examples of military incompetence or bad luck frequently combined to render good and timely information useless.
The Imperial War Museum houses the UK's only permanent exhibition devoted to UK espionage. Among the many fascinating artefacts are relics from the missions of SOE. Click on this link to find out more.
The Princess Royal (Princess Mary) visits a Y Station operations room at Kedleston Hall, Derbyshire in 1943. Picture courtesy the National Trust.
Bletchley Park, or Station X, may be the most famous wartime location in the battle to monitor and decipher enemy ciphers, but the UK also had several secret locations, known as Y stations. Such places undertook the important business of monitoring and listening to enemy wireless traffic.
Nevertheless, the high priority the Allied powers gave to intelligence gathering - particularly the efforts of Bletchley Park in its attack on the German 'Enigma' codes - was rewarded by allowing British and US leaders to read many German military orders and assessments before the intended recipient.
By the end of the war, good intelligence was one of the reasons why increasing numbers of U-Boats were sunk and the Battle of the Atlantic was won by the Allies.
In contrast, German efforts to penetrate Britain's vital strategic and technical planning and developments proved to be inept and easily detected. Captured spies were tried and most, if not all, were executed.
One unlucky German spy was the last man to be executed at the Tower of London. Click on this link to find out more.
Whatever comfort this may have brought to the British government was quickly dispelled when the Second War was seamlessly replaced by the Cold War. The Soviet Union ended any belief that the country's secrets were secure from comprehensive and sustained attack.
Click here to explore the secret world of SOE and WWII Special Operations on the web.