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. The colonel, disguised as a peasant, goes to gather information while Franco swims across the River Piave with the famous caimani unit (named after the South American alligator) and reports back to Italian headquarters. In one of his reconnaissances the colonel is mortally wounded. Lucilla, helped by a young crippled peasant, carries on with the dangerous mission. In the end the young peasant is killed, and Lucilla is captured by the Austrians and sentenced to death. At the eleventh hour she is rescued by the Italian cavalry.