Repeated announcements that the curtain will soon go up for opera's second act ("Curtain. Curtain going up. Second act") also signals a significant moment in Lisa's life - Stefan is about to re-enter her life. The calls sound as Lisa and Johann ascend the staircase to their opera box. She overhears a group of people next to the upstairs banister speaking about an aging, hedonistic Stefan Brand who has been viewed below them. Lisa joins them (in closeup) to look down from the balcony (in another camera angle of Stefan far below - again with another woman) to see him for the first time - seven years after conceiving his child:

First Man: He returned last week.
Second Man: A concert tour?
Woman: Pleasure trip most likely.
First Man: The way he's burning himself up, it's a wonder he's still alive.
Man (offscreen): Ten years ago, he showed great promise. Too bad. With that talent, he could have been a great pianist.
Second Man: Perhaps talent is not enough.
Woman: Perhaps he has too many talents.

Her husband stands next to her as she gazes down at the object of her sexual longing. In her opera box with her husband, she looks around warily into the dark. With a trembling voice-over, she worries about Stefan's presence in the opera house - when realizing that he may re-discover her:

Suddenly in that one moment, everything was in danger, everything I thought was safe. Somewhere out there were your eyes...and I knew I couldn't escape them. It was like the first time I saw you. The years between were melting away.

Using the excuse of having a headache, she leaves the box to lie down for a few moments, but then decides to leave the opera house. She descends the long spiraling staircase and calls for her carriage. At the side entrance as she nervously paces back and forth while waiting, Stefan's voice is heard off-screen just before their fatal reunion. Jaded and embittered and having given up on his music (never fulfilling his youthful promise), his rootlessness, neediness and loneliness cause him to reach out to her. Pretending to vaguely remember her, he begins his passionate art of seduction - of a married woman now - all over again. As she departs, he pretentiously asks her where she fits into his life - "among all others" - by asking: "Who are you?":

Stefan: Excuse me. You must realize where there is a pursued, there must also be a pursuer. I've seen you somewhere, I know. I followed you upstairs and watched you in your box. But I couldn't place you, and I had to speak with you. Oh, I know how this sounds. I assure you in this case it's true. You believe that, don't you?
Lisa: Yes.
Stefan: Thank you. (He gently puts his hand on her arm.) Now please, just a moment.
Stefan: Is there anyplace we could have met, that I might have seen you? Like one of my concerts? It must have been some time ago. I haven't given any concerts in Vienna lately - or anywhere else for that matter.
Lisa: You don't play anymore?
Stefan: Oh, it's not quite as final as that. I always tell myself I'll begin again next week, and then when next week comes, it's this week, so I wait for next week again.
Lisa: What are you waiting for?
Stefan: That's a very disturbing question.
Lisa: My carriage.
Stefan: You can't ask such a question and just walk away. I have a feeling - please don't think I'm mad. I know it sounds strange, and I can't explain it, but I feel that you understand what I can't even say, that you can help me. Have you...have you ever shuffled faces, like cards, hoping to find the one that lies somewhere just over the edge of your memory, the one you've been waiting for. Well, tonight when I first saw you and later when I watched you in the darkness, it was as though I had found that one face among all others. Who are you? (Lisa turns to leave and walks away.) Promise I'll see you again. I must see you.
Lisa (while turning her head slightly and looking back, but continuing to quickly walk away): I don't know.

Lisa's carriage pulls up, and she gasps when she sees Johann sitting inside waiting for her. During their carriage ride home, he is slightly angered and heartbroken by her romantic powerlessness and her deluded justification for returning to Brand. By allowing her heart to dictate the course of her life, she threatens to dash their own marital hopes of happiness:

Lisa: Johann, you don't think I wanted this to happen.
Johann: No. (Pause) What are you going to do?
Lisa: I don't know.
Johann: Lisa, we have a marriage. Perhaps it's not all you once hoped for, but you have a home, and your son, and people who care for you.
Lisa: I know that, Johann. I'd do anything to avoid hurting you, but I can't help it.
Johann: And your son, you think you can avoid hurting him?
Lisa: He won't be harmed. I'll see to that.
Johann: There are such things as honor and decency.
Lisa: I told myself that a hundred times this one evening.
Johann: You talk as though it were out of your hands. It's not, Lisa. You have a will, you can do what's right, what's best for you, or you can throw away your life.
Lisa: I've had no will but his, ever.
Johann: That's romantic nonsense.
Lisa: Is it? Johann, I can't help it. I can't. You must believe that.
Johann: What about him? Can't he help himself either?
Lisa: I know now that he needs me as much as I've always needed him.
Johann: Isn't it a little late for him to find out?

In their study, while standing in front of dueling pistols and swords hung on the wall, Johann threatens to duel her lover:

Johann: Lisa, if you do this, you can never turn back. You know that, don't you?
Lisa: Yes.
Johann: I warn you...I shall do everything in my power to prevent it. (She slowly walks away toward the staircase.)

Lisa enters her upstairs bedroom where her young son is sleeping in her bed. She bends down and kisses him on the forehead, wakes him up, and helps him to his own room. She tucks him into his bed:

Stefan: You're wonderful.
Lisa: You're wonderful. You've always been a wonderful boy. You don't know how much that's meant to me to be so proud of you always.

Her voice begins to break as the boy falls asleep. She turns and goes to the window, opens it, and then breaks into tears, burying her face in the curtains. The scene fades to black.

At the train station, Lisa and young Stefan enter a train compartment. He is traveling to the village, where she will join him in "two weeks" (reminiscent of the "two weeks" Stefan promised to Lisa before his long absence.) One of the conductors moves them to another compartment, because theirs was "quarantined," but they are not told that the carriage was infected with deadly typhus. Just before the boy departs, she takes his hands:

Lisa: Stefan...you know how I...I've longed so for your being home, and now I can't even explain. Maybe someday very soon, I'll be able to give you a reason...but now you'll just have to believe that it's for your own good.
Stefan: I don't mind going back so much, especially when you're coming so soon.

As the train pulls away, they wave goodbye to each other as steam from the train curls around her. When the train has fully passed, she walks slowly into the station, deep in thought. She passes a few groups of people talking about "a case of typhus...on the train," as a man on a stretcher is carried away.

Revisiting locales where she spent time with Stefan (recalling earlier scenes when she was in her youth), she walks to the restaurant where she spent part of an evening with him. Informed that he is "no longer a customer...for some time," she is directed to the Ritz. Outside, she looks up at Stefan's lighted upper-story window and realizes that he is there. After purchasing not just a single white rose but a large bouquet of white roses from a flower seller where he "still has nice flowers left," Lisa walks across the street to his iron gate and courtyard outside his apartment, where she stands and rings the bell to enter. A light from inside illuminates her.

After an abrupt long shot of her from the far end of the street as she enters the gate, a quick pan to the right reveals her husband Johann has been observing her from the window of his carriage - without her knowledge. Convinced of her infidelity, he tells his driver: "Now, you can drive on." After leaving her husband, she has emphatically risked everything in her society by offering herself to Stefan one last time and renewing her romantic pursuits of her youth.

In harsh shadows, Lisa climbs the stairway to his apartment, where she rings the bell and is allowed to enter. The servant John escorts her into Stefan's study, where she smiles and remembers the way it was years before when she was a young girl. Stefan also smiles when he finds her there, to his happy surprise:

Stefan: I thought last night (He turns up the light.) the darkness...might have played tricks on my eyes. You're even more beautiful than I imagined.
Lisa: Very kind of you.

He calls to John to go out to the corner store and buy a few things for a late supper, and then tells her: "John makes life possible for a hopelessly single man." Lisa has stood up to admire a terra cotta bust of an ancient Greek goddess. Lacking commitment to any earthly woman, Stefan also romantically idealizes a goddess that could make his life more meaningful:

Stefan (looking at the statue): She fascinates you, too. You remember the Greeks built a statue to a god they didn't know, but hoped someday would come to them...Well, mine happens to be a goddess.
Lisa (smiling): And you never found her.
Stefan: For years, I never woke in the morning, but I said to myself, 'Perhaps today she will come...and my life will really begin.' Sometimes it seemed very near. Well, now I'm older and I know better.

Lisa asks why he doesn't play the piano anymore, a piano whose keyboard is often locked up, and he explains:

One night, I came back to this room. I'd given a concert like all the others, not better, not worse. Afterwards, they'd said all the usual things, the things you say when you're not really convinced. So I happened to look in the mirror, this one here. The young prodigy was no longer young, certainly wasn't prodigious.

Then he walks toward her and puts his arms around her, lifts the veil that covers her face, and kisses her passionately:

Since then, I found other things to do, more amusing things. I knew last night, didn't you?

As he goes to pour glasses of champagne, the self-centered, self-indulgent pianist talks about his lastest American trip. He reverts to a formulaic attempt at seduction, while she attempts to interrupt him with a serious thought:

I came here...I have something to tell you...It has to do with us.

As he prepares their drinks off-screen during the casual seduction, it slowly dawns on her that the unworthy man doesn't even remember her and that she is only another in a long line of adored, but forgettable female conquests.

Stefan: So you travel a great deal? (She doesn't answer) Well, I suppose you won't believe me, but I couldn't get you out of my mind all day.
Lisa (faintly crying): No, I don't believe you.
Stefan: If only I had known you were coming, but to tell you the truth, I never dared hope you could arrange it so soon. But you're so clever. You managed so beautifully. You know, you're a very strange woman.
Lisa: Am I?
Stefan: Something you said last night keeps running through my mind.
Lisa: What was that?
Stefan: I'll tell you in just a moment as soon as I get the ice...Are you getting lonely out there?
Lisa (her voice cracking): Very lonely.

Although her life has been completely dominated by him, she leaves him after unmasking his pretense and juvenile duplicity. [For the third time in the film, Lisa departs from him.] Lisa slowly walks over to the chair where she had laid her purse and coat. She leaves the room where the roses she had brought lay in a pile on a chess table next to a vase, as her voice-over ruminates:

I'd come to tell you about us, to offer you my whole life, but you didn't even remember me.

On her way down the staircase of his apartment, she passes John with a tray of food covered with a white cloth. She leaves the building and enters the darkened street, where she is approached by a drunken man in uniform, who asks to take her somewhere: "anywhere at all. Makes no difference." Bursting into tears, she glances up at Stefan's lighted window, hurries away and walks aimlessly in the streets:

I don't remember where I went. Time moved past me not in days or in hours, but in the distance it put between us. When I could think again, I went to my son, but it was too late.

The scene dissolves to a crucifix hanging on a bare wall, with a burning candle next to it, and then her son's deathbed:

He died last night of typhus without even knowing I was there.

Lisa is viewed in the hospital, penning the last lines of the unsigned, long letter that has been the substance of the film's many flashbacks and voice-overs. Her strength fails her as she writes three phrases beginning with "if only" - startingly, she still professes her undying love for him - a lifelong, bleak fantasy of hers!:

Now I'm alone. My head throbs and my temples are burning. Perhaps God has been kind, and I too have caught the fever. If this letter reaches you, believe this - that I love you now as I've always loved you. My life can be measured by the moments I've had with you and our child. If only you could have shared those moments, if only you could have recognized what was always yours, could have found what was never lost. If only...

The final flashback scene blurs and dissolves into the final page of the letter (with letterhead from St. Catherine's Hospital) - it ends abruptly with a black blot of ink [indicative of her death] on the word 'only.' It is unsigned and unaddressed, but clipped at the bottom of the last page is a typewritten and handwritten postscript provided by a nun at Lisa's deathbed:

(Typewritten):
This letter was written by a patient here. We believe it was meant for you as she spoke your name just before she died.

(Handwritten):
May God be merciful to you both.
Mary Theresa
Sister in Charge

The camera moves in for a closeup shot of Stefan's face, accompanied by violins. Tears glisten in his eyes as he comes to the final words of the letter, and makes an effort to remember the woman. A misty montage of flashbacks play - but they are only fragmentary scenes of their romantic encounters:

- Stefan lifting the veil from her face during her final visit
- Stefan walking toward her as she stands on his street corner
- Stefan and Lisa dancing
- Stefan and Lisa (with her single white rose) in the carriage
- Lisa being presented with a caramel apple in a shuttle box by a carnival worker
- Another image of Stefan and Lisa dancing
- Lisa kneeling next to the piano as Stefan plays

Stefan is overcome with grief, exhaustion, and horror at his desk by the flood of images - a tear tracks down his cheek as he covers his eyes.

John reminds him of the time approaching the duel, as church bells chime in the distance. John nods recognition of the woman when Stefan asks him to recall her:

You remembered her?

And then the mute servant John, who brought the letter, gives the 'unknown woman' a name. He writes 'Lisa Berndl' on a piece of paper - functioning as Lisa's signatory. Stefan repeats her name twice: "Lisa, Lisa" - his first knowledge of the unknown woman's name, although her name has been used repeatedly throughout the flashbacks! Even after her death, he cannot relieve her longing by recognizing her.

A carriage pulls up on the cobblestone street outside in the dawning light - the grief-stricken husband Johann, his lost love Lisa's husband, has challenged Stefan to a duel. After reading the letter, Stefan changes his mind about leaving the city and decides to boldly meet his opponent and face an inevitable death. When he is ready, he turns toward the bouquet of white flowers (now arranged in a vase) that Lisa had brought to their final meeting. As he takes one white rose and pins it on his lapel, he remembers the final sentence in Lisa's letter, the final line in the film spoken in Lisa's voice - the most poignant "if only":

Oh, if only you could have recognized what was always yours, could have found what was never lost.

In one of the most touching scenes in film history, as Stefan leaves his apartment, he stops and looks back from the iron gate to the door, where he tries to remember Lisa's image as he first saw her - a wraith of a girl, only fourteen, shyly holding the door open for him. He smiles sadly at the vision that appears and then fades from view. He closes his eyes and looks down, sensing the loss and all that Lisa had sacrificed for him. Then, he enters the waiting carriage (another carriage scene, one that loops back to the beginning of the film and reverses the action) - the carriage that will possibly take him to his own death in a suicidal (but redemptive) duel with Lisa's husband - who is an excellent duellist ("an excellent shot").