It is 1917, the period of the First World War when Italy is fighting Austria-Hungary. Lucilla di Torrebruna leaves high school and returns to her father, a colonel in the cavalry, who lives at San Donà di Piave. He tells her that his next wife is going to be Helène, a foreign lady. Lucilla takes the news calmly, pleased that her father has found happiness, although she receives a somewhat cold indifference from her new stepmother. Franco, a young man from Trieste and an old college friend of Lucilla with whom he shares a mutual affection, gets through the Austrian lines and enrols with crack Italian corps, the bersaglieri. After the Battle of Caporetto, San Donà is occupied by the enemy and the Torrebruna villa becomes the Austrian headquarters. Now Helène reveals her true colours - she is an Austrian spy. Franco and the colonel organize counter-espionage operations.