The following synopsis is reported by Variety:
The story takes place in 1943, as the Allies prepare to launch an all-out assault on Nazi-held Europe. They face a seemingly impossible task – to protect a massive invasion force from entrenched German firepower and avert a potential massacre. Two intelligence officers, Ewen Montagu (Firth) and Charles Cholmondeley, dream up an inspired disinformation strategy, centered on the corpse of a dead man.
This sounds pretty interesting and like it has a solid cast and crew behind it. The fact that it will tell a true story to give us even more insight into WWII sounds really appealing.