In 1705, the French entered Pianezza and the inhabitants fled in panic. Some soldiers mistreat a defendeless girl, Maria, who escapes through an underground cave leading into the castle. She spies on the French and then slips away to the Piedmontese camp of duke Amedeo di Savoia. There she draws her route on paper, which is delivered to the general. Followed by soldiers, Maria guides them into the castle, where the French are surrounded and defeated.