In 1915, at a time when Austria, Germany and Italy were supposed to be allied, two Italian brothers are pitted against each other not only for divisive politics but also because they love the same girl, Anna, the niece of an Austrian lieutenant.

Anna's Sin tells the story of a black American actor (Johnson) who falls in love with a white woman playing Desdemona to his Othello on the Italian stage.

The Milanese Luigi Pizzigoni, photographer, and the Neapolitan barber Peppino Avallone, resident in Milan, are opponents in the sports field. The first is a proud Inter supporter, while the second is from Napoli.