By 1942 German scientists seemed to have an insurmountable head start over their Allied counterparts in developing an operational atomic bomb. Contributing to the Nazis' nuclear lead was their access to large quantities of an ingredient crucial to atomic experimentation: "heavy water," the world's largest producer and stockpiler of which was the fortresslike Vemork factory in occupied Norway. Allied hopes of stalling the Nazi nuclear program soon focused on sabotaging the virtually impregnable cliffside plant-a suicidal mission by any estimation. But a team of brave Norwegian exiles, trained in Britain by Special Operations Executive, infiltrated their homeland and, while hiding and eking out an existence in the wilds, awaited the opportunity to launch one of the war's most daring commando raids.