 In 1866, in the spring of Venice, an underground rebel movement against the Austrian occupation in Italy is getting stronger. The married Countess Livia Serpieri (Alida Valli) sees her cousin challenging the Austrian Lieutenant Franz Mahler (Farley Granger) for a duel in the opera and being arrested and sent to the exile for one year. When she meets Mahler, she first hates him but after a night together walking along the streets in Venice, she falls deeply in love for him, becoming his mistress. When the war starts, she moves to her property in the country missing Mahler. When he visits her during a night, she forgets her principles, decency and betrayals her cause with cruel and tragic consequences.