The year is 1862, one of those periods, rare in Italian history, without any wars. The young Kingdom of Italy does not pay its officers to fight, but merely to train others to do so, and at the first possible opportunity.

In a Piedmontese town, Giorgio Bacchetti, captain in a cavalry regiment, who had fought fairly heroically in Crimea and on the Spromonte, meets Clara who is beautiful, married and the mother of a child. Giorgio and Clara immediately fall in love. Their love is happy and similar to that of many others; one is justified in talking about it merely in contrast to the unhappiness which Giorgio causes another woman.

He is transferred to a garrison near the frontier and so is painfully separated from Clara, to whom he sends letters every day. Like all the other officers, Giorgio dines at the colonel's table. One of the places is always laid but left empty; it is meant for the colonel's young cousin, who is almost always ill and goes down to dinner very seldom. She stays in her room where she spends her time reading, playing the piano and curing her incurable illness.

Giorgio's curiosity is aroused, and he is vaguely troubled by that somewhat mysterious presence which hangs like a cloud over the whole house and, in a way, over the whole garrison.

At last, one day Giorgio manages to see the girl and is struck speechless: the girl, who glorifies in the name of Fosca, is extremely ugly. Whereas Clara seems to be a symbol of all that is harmonious in a woman, Fosca appears to personify all possible lack of harmony in any human being. And she is already desperately in love with Giorgio ...