Those who aren’t film students might not know the definition of “MacGuffin”. In the cinematic world, a “MacGuffin” is the particular narrative element that drives a narrative. It is pertains very little to the story, but without it there would be no story. Therefore, the MacGuffin is what keeps the audience engaged and asking questions to the end of the picture.
In Notorious (1946), the MacGuffin is a particular uranium ore being mined in the mountains of Brazil by the Nazis. Their plan is vague at best but roughly amounts to reorganizing and weaponizing in the Western Hemisphere after the war to threaten the United States. In true MacGuffin style, it doesn’t really matter what they are doing with the uranium but the fact that they are doing something with it is the key driver. Notorious was the first prominent nuclear movie released after the bombing of Hiroshima and Nagasaki, so even the non-specific mention of the nuclear fuel uranium (and its implied reactions) combined with the on-screen reactions between Ingrid Bergman and Cary Grant (another energy movie frequent flyer) propelled this picture into critical and popular success.