On the film, a German band marches through a small town, singing the virtues of the Nazi doctrine. Donald is awaken by the noise and taken by the musicians to an ammunition factory. As one long day of forced work passes, he becomes little more than another slave of the totalitarian system, with no choice but obeying till he falls, suffering a nervous breakdown. However, he finds out that it has all been a nightmare and he is still living on the land of liberty.
After the short was produced, its theme song was made even more popular by Spike Jones and his City Slickers. Der Fuehrer's Face won the 1943 Academy Award for Animated Short Film.
http://disneyshorts.toonzone.net/years/1943/derfuehrersface.html (Includes a link for downloading the entire short)