Passion opens in Milan with two young lovers, naked in bed, singing of their ardor (Happiness). The handsome young soldier Giorgio breaks their reverie by telling his beautiful mistress Clara that he is being transferred to a provincial army outpost. As Giorgio prepares to depart, he celebrates Clara's beauty.
In the mess hall at the outpost, Giorgio meets Colonel Ricci, the regiment's commanding officer, and Dr. Tambourri, its attending physician. Giorgio's reputation for heroics has preceded him. He thinks longingly of Clara (First Letter), and Clara thinks longingly of him (Second Letter). Giorgio's thoughts are rudely interrupted by a frightening scream. The Colonel tells him not to worry - it's his sickly cousin Fosca. He tells Giorgio that reading is Fosca's only passion; Giorgio offers to lend her some books.
At this army post so far from the city, the sensitive Giorgio feels increasingly out of place (Third Letter). He starts to form a friendship with the Doctor, who describes Fosca as "a kind of medical phenomenon, a collection of many ills." Her illness manifests itself in hysterical convulsions and exposes her fearlessness, her pain and her need for connection.
In the mess hall, Giorgio opens another letter from Clara (Fourth Letter). As he does so, a shadowy figure behind him descends a long spectral staircase.
Clothed in a drab green dress, her body stiff with pain, the unattractive Fosca introduces herself to Giorgio (I Read). She gives him a bouquet of flowers, then collapses in a hysterical faint. Giorgio is stunned (Transition).
We are transported from the mess hall to the neglected garden of a nearby ruined castle, where the Colonel asks Giorgio to lend Fosca his arm. While Giorgio and Fosca stroll, he thinks of Clara. Giorgio and Clara sing a duet, a letter he has written to her. Fosca accuses Giorgio of cruelty (Garden Sequence).
As the days pass, Giorgio and Clara exchange letters about Fosca. Clara warns him to keep his distance. In fact, Fosca has already developed a dangerous obsession. At dinner that night, she gives Giorgio a letter declaring her feelings, but he chooses not to read it. She grabs his hand beneath the table and refuses to let go. Giorgio immediately asks the Colonel if he can have a five-day leave to go to Milan. With some hesitation, the Colonel agrees (Transition).
The following morning, as Giorgio prepares to go, Fosca ambushes him and throws herself at his feet, avowing her love. He calms her, but not before agreeing to write her as soon as he arrives in the city.
While Fosca, sitting alone in her parlor, sings tearfully from the letter Giorgio has sent her, Clara and Giorgio, back in their little room, sing of their love for one another (Trio).
Upon Giorgio's return to the camp, Fosca coldly reproaches him. She questions him about his affair with Clara and learns that her "rival" is married. In a sharp exchange, they both agree to sever all ties.
Three weeks pass with no contact between them (Transition), but just as Giorgio is beginning to think that he is finally free of Fosca, Dr. Tambourri informs him that she is dying. Giorgio's rejection of her, the Doctor says, has accelerated her disease; Giorgio has a duty to go to her now and offer a few words of hope. Giorgio reluctantly agrees.
He enters Fosca's bed chamber, and she implores him to lie beside her on the bed while she sleeps. At daybreak, Fosca asks him for a favor before he leaves: "Write a letter for me." He agrees, but the letter she dictates is a fantasy one from Giorgio to herself (I Wish I Could Forget You). She then encourages him to give her a brotherly peck on the forehead, which she tries to turn into a passionate kiss. He hastens from the room.
The other soldiers gossip about Giorgio and Fosca while playing pool (Soldiers' Gossip).
Colonel Ricci reminisces about his cousin to Giorgio, and we move back into the past to the days when Fosca still had illusions about her looks, expectations about her prospects. We learn how she fell in love with a handsome gigolo, how he married her for her money and how he abandoned her (Flashback).
Clara, meanwhile, has written Giorgio another letter (Sunrise Letter), which he reads as he makes his way to a desolate mountain heath. No sooner has he sat down than Fosca appears. When she tries to kiss him, he lashes out at her in anger (Is This What You Call Love?). She collapses, and he is so angry that he almost abandons her in the pouring rain.
The other soldiers gossip about Giorgio and Fosca coming back to camp soaked to the skin (Soldiers' Gossip).
The rain, the ordeal of getting Fosca back to camp, perhaps the exposure to Fosca's contagious emotions - all have conspired to give Giorgio a fever. The Doctor attends to him and informs him that he is being sent home to Milan to recuperate (Transition).
Clara happily anticipates his sick leave (Forty Days).
Giorgio, still in poor health, boards the train for Milan, only to be followed once again by Fosca. They have an angry exchange. Giorgio informs her that he will never love her. He begs her to give him up. She explains that she cannot (Loving You). Giorgio is moved by the force of her emotions; ultimately, he takes her back to the camp (Transition).
Startled to see Giorgio back so soon, the Doctor warns him that he must stop seeing Fosca, that she poses a threat to his mental and physical health. The Doctor will have him permanently transferred if he doesn't take his leave. The other soldiers gossip about Giorgio's 40-day sick leave (Soldiers' Gossip).
Giorgio goes to Milan and tells Clara he will not take his full leave - a decision that provokes Clara to question him jealously about Fosca. Giorgio responds by asking Clara to leave her husband and run away with him, but she demurs.
Giorgio returns to the post and attends a Christmas party there. The festive mood is broken by the Colonel's receipt of orders recalling Giorgio to military headquarters. To the astonishment of the Colonel and the other soldiers, the news sends Fosca flying into Giorgio's arms, begging him not to leave. She rushes from the room in tears.
The Colonel, outraged, orders Giorgio to wait for him as he leaves the room to attend to his cousin. Stunned, Giorgio reads Clara's latest letter (Farewell Letter).
The Colonel returns from Fosca's bedroom, having discovered the letter Fosca dictated to Giorgio. He accuses Giorgio of leading his poor cousin on and challenges him to a duel.
That evening, Giorgio returns to Fosca's bedroom and finally surrenders to the passion she has awakened in his heart. He loved Clara, he tells her, but no one has ever loved him like Fosca (No One Has Ever Loved Me). Knowing the act of love might very well kill her, Fosca leads Giorgio to the bed, where they embrace, their passion consummated at last.
The duel takes place the following morning. Giorgio wounds Colonel Ricci, then responds with a scream - a scream eerily reminiscent of Fosca's earlier hysterical outbursts.
Several months pass, and Giorgio, who has been recovering from his breakdown, is sitting alone at a small table in a sanatorium. A nurse enters and hands him a letter and a box. The letter is from the Doctor. Colonel Ricci, he writes, recovered from the wound he received in the duel; Fosca died three days after her night of passion. She has left Giorgio a letter and a box containing some of her possessions, which the Doctor has enclosed. Dreamlike, the other characters in the story appear, as Giorgio begins reading from Fosca's final letter. Her voice gradually joins his. She then appears beside him, and together they sing of their love (Finale).
The company slowly walks off, followed by Fosca, leaving Giorgio alone at his table.
- Michiko Kakutani