During the last three days before D-Day, all preparations have been made, but one essential element remains uncertain: the British weather. The meteorologist James Stagg is tasked with giving the most important forecast in the history of mankind. The wrong weather conditions can ruin the planned overseas attack, while any delay increases the risk of unmasking by the Germans. The final decision rests with the Allied supreme commander Dwight D. Eisenhower. With only Kay Summersby as his personal confidante, he has only a few hours to decide the fate and lives of millions of people.