The story of Bletchley Park's role in cracking of German codes on and in the build-up to D-Day is being told for the first time.
A new exhibition at the once-top-secret location highlights how intelligence was built ahead of the landings - not just through the interception of messages but also through spies on the ground, prisoners of war and aerial surveillance.
Recently unclassified documents show how codebreakers at Bletchley Park played a key role in producing a very detailed picture of northern France prior to D-Day.
"In 1942, Bletchley sets up a thing called the 'Western Front Committee'," explained David Kenyon, research historian at Bletchley Park.
"They met once a week and their role was that all representatives on the committee would bring intelligence that they think would be relevant to an invasion.
"Their first task [was to] understand where all the units are, how many tanks they've got, how many troops, they've got."
All intelligence obtained in the committee was recorded and stored in 'card index systems' which remain in good condition.
One line on one card entry recalls "German preparation for the Second Front", which was sent from Stockholm to Tokyo on 5 May 1944, around a month before D-Day.
A major part of the exhibition is an immersive 12-minute film, played across a 22-metre screen broken down into smaller viewing panels.
"This film has allowed us to show visually but also in a really layered-manner through media such as illustration, animation, archive/footage to really pull out the different strands," said exhibitions manager, Erica Munro.
"There's obviously the different sources of intelligence that fed into the intelligence picture."
The intelligence obtained meant troops landing on the Normandy beaches had detailed information about the whereabouts of the enemy, their firepower and numbers.
The recording of intelligence did not stop on D-Day itself.
Other files also show the intercept of messages on 6 June 1944.
One source recalls a scene involving 30 paratroopers and 40-80 aircraft circling, resupplying enemy soldiers before flying over the sea.
Organisers hope 'D-Day: Interception, Intelligence, Invasion', which opens on Thursday, will underline how decoding and other intelligence helped shape the planning and execution of D-Day.
Watch: Forces News spoke to Edward Simpson, a mathematician who became a World War Two codebreaker working at Bletchley Park alongside Alan Turing.