The D-day landings - the fate of 2.5 million men, 3000 landing craft and the entire future of Europe depends on the right weather conditions on the English Channel on a single day. A team of Allied scientists is charged with agreeing on an accurate forecast five days in advance. But is it even possible to predict the weather so far ahead? And what is the relationship between predictability and turbulence, one of the last great mysteries of modern physics? Wallace Ryman has devised a system that comprehends all of this - but he is a reclusive pacifist who stubbornly refuses to divulge his secrets. Henry Meadows, a young maths prodigy from the Met Office, is sent to Scotland to discover Ryman's system and apply it to the Normandy landings. But turbulence proves more elusive than anyone could have imagined and events, like the weather, begin to spiral out of control.