No, not Sweeney Todd. Not Assassins. Those, while disturbing on many levels, are explicable. Sweeney Todd is about corruption, exploitation, innocence and ignorance, and the evil in the human soul and society; Assassins is about the monster people have made of the American Dream from their own impotence and futile anger.
This one is a simple love story, but it can't be unraveled so neatly. And it's left profoundly disturbing. Other Sondheim fans know that of which I speak.
When Passion opens, Giorgio (a mid-to-late nineteenth-century Italian soldier) is happily involved with a married woman, more than content with the love they have. He continues to sing its praises even when he gets transferred to a strange little military town and meets Fosca, the cousin of his commanding officer.
Fosca is ugly, sickly, and most of all (as we discover) needy. She quickly becomes obsessed with Giorgio, to the point of practically stalking him. She obsesses over him. She follows him everywhere, refuses to allow him to have a moment without her. She gets sicker and sicker, and she continues to be desperate, needy, possessive, and jealous at Giorgio. She is thoroughly unlikable, and this much, I am sure, is intentional.
She is also bizarrely manipulative--at one point, she begs Giorgio to write a letter for her, and as she dictates it, it becomes clear that she's writing her own fantasy letter from him to her, talking about how although he cannot love her, he wishes he could.
We eventually learn that Fosca was once taken advantage of--married for her money, then abandoned as soon as that ran dry--by a con man pretending to be aristocracy, and since then, she has been sick. This still doesn't make her very sympathetic, and I don't think it's supposed to.
Giorgio tries to avoid her, and mostly he succeeds. Then one day she follows him out somewhere and clings to him some more, and he finally snaps, telling her off for this insanity she calls love. She collapses, and it starts to rain. Giorgio starts to walk away, but then comes back, picks her up, and carries her home.
Of course, because of this, he becomes sick. He takes a brief sick leave (which was supposed to be much longer, but he refuses to take that much leave) and spends it with his lover, Clara--but he feels differently about their love now. He begs her to leave her family and come with him, because reason doesn't matter in love. She tells him she can't. He refuses to believe this.
Shortly after he goes back to duty, he receives a letter from Clara telling him that if he doesn't shape up and start being reasonable again, their relationship is over, because he's changed. He turns the same words he used on Fosca earlier, in her terrible obsession, on Clara--is this what you call love?
At that point, Fosca's cousin, the Colonel, finds the letter from earlier in the show. Of course he doesn't realize the story behind it; he only sees a letter from Giorgio to Fosca, in Giorgio's handwriting, declaring his feelings for her. He accuses Giorgio of leading her on and demands that they duel the next morning.
Giorgio slips into Fosca's room that night and declares his love for her. She begs him to make love to her, although they both know she is too fragile to withstand the physical exertion; he refuses at first, fearing for her health, but she convinces him: "to die loved is to have lived." They do the deed.
In the morning, Giorgio and the Colonel duel. Cut to several months later: Giorgio is in bed (the same prop as Fosca's sickbed) reading a letter from Fosca's doctor. Fosca died three days after the last time Giorgio saw her. The Colonel was hurt, but survived. Giorgio has been recovering from his wounds all these months, and it is implied that he is still recovering--perhaps chronically ill like Fosca?
Fragments of the previous songs are sung in pieces by various cast members. Fosca shows up on stage, and she and Giorgio sing about love. End.
And there you have it: one fucked-up musical. The really disturbing thing is that all of Sondheim's previous musicals, with their complex, somewhat cynical, but much more palatable ideas about love...were written before he had ever really experienced love. This one was written afterwards. Crazy faggot.