Wing Commander Roy Grant (Cliff Robertson) is the commander of 633 Squadron of the Royal Air Force, flying de Havilland Mosquitoes on raids against the German forces in occupied Europe during World War II. In the run up to D-Day in 1944, 633 Squadron is given a highly dangerous mission to bomb a German rocket fuel factory in Norway. Just to complicate things, their target is not the factory itself but a rock face overhanging it that will collapse onto the factory and destroy it. To do that they need to fly at extremely low level along the length of a Norwegian fjord and under the guns of the German defenders. To increase their chances slightly, the Norwegian resistance, led by Lieutenant Erik Bergman (George Chakiris), will attack the German anti-aircraft guns along the fjord just before the attack takes place. Grant isn't that enthusiastic about the plan, but while the squadron trains for the mission, he at least has the distractions of Bergman's attractive sister Hilde