Passion opens in Milan during the 1860s. The show starts with a couple (young soldier Giorgio and his lover, the married Clara) "bonking on stage" (quote Michael Ball). Afterwards Giorgio has to disturb the happy moment (Happiness) by telling Clara that he got transferred to a Northern Italy outpost.
At the outpost he meets Colonel Ricci, the commanding officer, and the physician Dr. Tambourri. During dinner, Giorgio thinks about Clara. His reverie is suddenly disturbed by a loud scream. Colonel Ricci explains that this was just his sick cousin who lives with the Colonel, as she has no family of her own.
Some weeks later Giorgio meets the unattractive and bizarre Fosca for the first time (Fosca's entrance). Very soon Fosca becomes obsessed with Giorgio. For her it has been love at first sight and therefore she starts to take any chance to meet Giorgio.
After just a month, Giorgio asks for permission to leave for Milan for five days as he is urgently needed. In fact he just wants to escape Fosca and that dreary place for a few days, as he is very unhappy there. The following morning he is on his way to the train, while Fosca pleads with him to stay. She will not let him go until he promises to write to her the next morning. While he has a nice time with Clara, Fosca receives his letter in which he explains, that he is in love with another woman and hat Fosca's love for him has no future.
After his return he has a talk with Fosca and she agrees not to see him again. Three weeks pass and Fosca keeps her promise not to see him. Meanwhile the Doctor tells Giorgio that Fosca is mortally ill. She wants to let herself die because of him. Therefore the Doctor begs Giorgio to visit Fosca. Reluctantly he agrees and spends the night lying by Fosca's side, as she wishes. After the night she asks him to write a letter she will dictate. It turns out to be a phantasy letter from Giorgio addressed to Fosca. Before he leaves she asks him to give her a brotherly peck on the forehead, but as he does she tries to turn it into a passionate kiss. He refuses and hastens out of the room.
Colonel Ricci tells Giorgio the story of Fosca's life. She was married to a marriage imposter who finally abandoned her (Flashback).
One morning Giorgio makes his way up to a mountain where he wants to read a letter from Clara (Sunrise Letter). When he sits down, Fosca suddenly appears. Giorgio gets very furious (Is this what you call love?). During the attack of insults Fosca collapses and a thunderstorm starts. Giorgio starts his way back to the camp, abandoning Fosca at first, but then he returns back and carries her back through the pouring rain, wrapped in his coat. After this trip Giorgio gets ill and is unconscious for about two days. To recuperate the Doctor sends him for a 40-day-leave back to Milan.
On the train he meets Fosca again who wants to follow him to Milan. He tries to persuade her, that she has to give him up, but she explains, that loving him is everything she has in her life (Loving you). Giorgio seems to be very touched, he wants to bring her back to camp before he makes his way to Milan again. He also wraps his blanket around her, as she shivers, although he is still very sick himself.
In Milan he finally asks Clara to leave her husband, but she declines. She accuses him of having fallen in love with Fosca. Giorgio finally decides not to take his sick leave and to return to camp. There he meets Doctor Tambourri who asks him to leave. The Doctor sees in Fosca a danger to Giorgio's mental and physical health. He threatens to have Giorgio permanently transferred, but Giorgio stays nevertheless, as he claims, that no one has ever loved him like Fosca.
During the Christmas party the Colonel receives an order that Giorgio is to be transferred back to headquarters. Fosca flies to his arms and begs him not to leave her. The Colonel is terrified and takes Fosca away. While waiting for the Colonel's return, Giorgio reads Clara's last letter (Just another Love Story). The Colonel reappears holding the letter Giorgio wrote some time ago for Fosca. He accuses Giorgio of leading Fosca on and challenges him to a duel.
Giorgio wants to see Fosca before this. They spend a first and last passionate night together (No one has ever loved me). The next morning Giorgio wounds the Colonel (Duel) and has a nervous breakdown. The next scene we find Giorgio having already spent several weeks at a sanatorium to recover from his breakdown. There he receives a letter (Finale) from the doctor who tells him that the Colonel's wound was serious, but not mortal and that Fosca died three days after they have seen one another for the last time. Furthermore he reads a letter that Fosca had written to him just prior to her death. In this letter she explains that she is now ready to leave the world for now she has experienced how it feels to be loved.