An attempt on the life of British high commander General Sir Lawrence MacKenzie-Smith by German intelligence, a chief strategist for the planned invasion of mainland Europe, spurs a search for a lookalike to impersonate him.
The situation gradually unravels when Lady Margaret, MacKenzie-Smith's wife, returns prematurely from a war bond tour in Canada to divorce him, having tired of his neglect and his womanizing.
However, after a wild regimental party, he is kidnapped by one of MacKenzie-Smith's mistresses and a traitorous officer, both Nazi agents, and is then flown to Berlin and interrogated about the Allies' plans for invading France.
Unable to convince his captors that he's a decoy and not the real MacKenzie-Smith, Ernie then blabs about non-existent invasion plots, which the Germans take as truth.
After a comedy of errors in a Berlin cabaret, Ernie disguises himself as a Luftwaffe pilot and boards a bomber bound for England, where he manages to parachute out.