Unable to sleep in the middle of the night, Alex arises from his bed early and learns that he has been fooled when he notices that the missing key has been replaced on his key ring. He begins to suspect that Alicia is spying on him. Exploring in the wine cellar, he also discovers the tampered 1940 wine bottle, the metal ore on the floor and glass shards from a broken 1934 wine bottle. Thinking that he has been double-crossed by Alicia in his own home - with the help of Devlin, his shadow is elongated on the checkboard floor of the main entry hall. Unsure of his next move, he slowly ascends the staircase and walks into his mother's bedroom. Guiltily crouched over next to her bedside "like a stupid schoolboy," he falls back on his mother's advice and consultation. He wakes her, pleads for her help, and confesses to her - with a most-famous line:

Sebastian: Mother, Mother.
Mme. Sebastian: Why are you up so early?
Sebastian: I need your help.
Mme. Sebastian: Something is wrong?
Sebastian: A great deal - Alicia.
Mme. Sebastian: I have expected it. I knew. I knew. What is it? Mr. Devlin?
Sebastian: [a high overhead shot] No. I am married to an American agent.

Alicia sleeps, completely unaware of her unmasking. Alex is anguished and hysterical with worry after discovering his wife's betrayal and fearing retaliation by his Nazi partners/conspirators. He is also personally distraught over how he was fooled by Alicia's "clinging kisses." With her suspicions now confirmed, the exultant Mme. Sebastian finds some consolation in her son's stupidity and convinces him to play along with Alicia's counter-espionage plan:

Mme. Sebastian: Yes, it is easy to see now. I knew but I didn't see. They picked her because of her father.
Alex: I must have been insane, mad. Behaved like an idiot, to believe in her with her clinging kisses.
Mme. Sebastian: Stop wallowing in your foul memories.
Alex: Then what do I do? There's nothing to do. I'm done, finished. They'll find out.
Mme. Sebastian: They won't find out.
Alex: They'll find out what I'm married to. Look what they did to Emil Hupka, Emil who did nothing. And I've betrayed them, I've bungled and there's no excuse. I'd do the same myself - kill the fool that betrayed them.
Mme. Sebastian: There's no need for them to find out.
Alex: Mathis is very sharp.
Mme. Sebastian: Yes. He dislikes you. But his criticism of your talents wouldn't go that far to imagine that you are married to an American agent. You are protected by the enormity of your stupidity - for a time.
Alex: Alicia, I'll take care of her myself.
Mme. Sebastian: No, not that way.

Shifting the villainous blame to herself, his autocratic mother proposes: "Let me arrange this one." Alex's steel-willed, authoritarian mother brutally proposes to liquidate agent Alicia by slowly, unsuspiciously, poisoning her to death [invading and violating her body with a foreign agent!], causing her to first become ill and then supposedly to die of natural causes:

Listen to me. No one must know what she is. There must be no suspicion of her, of you or me. She must be allowed to move about freely. But she will be on a leash. She will learn nothing further to inform. She must go, but it must happen slowly. If she could become ill and remain ill for a time, until... [The screen fades to black.]

In a number of scenes, Alicia drinks cups of coffee, slowly poisoned in small doses. Later in Prescott's company, she complains of light, painful headaches and dizziness. He proudly compliments her on their successful subterfuge - the sample that Devlin found was not sand but uranium: "That sand that Devlin brought in shows uranium ore so now we know what we're driving at. And your job from now on will be to try to help us find out where that sand comes from." She is also informed that her contact will change in a week's time, and that Devlin is being transferred to Spain - at his own request. With tears welling up in her eyes, Alicia interprets his change of locale as a personal affront (she has become "stale," uninteresting, and "routine" to him) and as an emotional desertion:

Mr. Devlin's been transferred to Spain...Well I guess he thought he was going stale here...I guess he thought he'd find Spain more interesting...Well, of course, it is more or less routine now.

As time passes after more emptied cups of coffee, she becomes weaker and weaker, barely able to punctually keep a rendezvous with Devlin in Rio at a park bench. During what she believes is their last meeting, she is exceptionally distant. He suspects that she is ill and "all mashed up," while she defiantly excuses her condition as another hangover from drinking [her thirsting for love turns to a thirsting for alcohol to escape the "dull" life he has commissioned her to have].

Devlin: It gets a bit lonely squatting on a bench all day.
Alicia: Yes, Rio can be a very dull town.
Devlin: What's new?
Alicia: Nothing. What's new with you?
Devlin: Nothing. Any domestic troubles about the other night?
Alicia: No.
Devlin: Any footprints in that sand yet?
Alicia: No, nothing yet.
Devlin: Just a social visit, huh?
Alicia: A little fresh air helps.
Devlin: You don't look so hot...Sick?
Alicia: No. Hangover.
Devlin: That's news! Back to the bottle again, huh?
Alicia: It sort of lightens my chores.
Devlin: Big party?
Alicia: Just the family circle.
Devlin: Sounds quite jolly.
Alicia: It helps life in a dull town.
Devlin: You ought to take it easy on that liquor.
Alicia: Don't you find Rio a little hard to take too?
Devlin: Not a bad town. You look all mashed up. It must have been quite an evening.
Alicia: It was.
Devlin: OK, if you want to play that way, go on and have fun. No reason why you shouldn't.
Alicia: That's right, Dev.

They each disguise their true feelings and concerns for each other, as Devlin pursues another official line of questioning. Despondent about his departure, she tries to get Devlin to express his bottled-up emotions, but he cannot see anything but an unrepentant alcoholic lush before him. She is annoyed when he doesn't mention his leaving, so she returns the midriff scarf he lent her when they first met in Miami. He quips: "Cleaning house, hmm?" Although still wobbly and "tight," she says "goodbye" (with an air of finality) to go "back home."

In a beautifully-orchestrated scene in Sebastian's living room, Alicia becomes aware of the plot against her - that she is being drugged with poison stirred into cups of coffee. In their company, when Dr. Anderson mistakenly picks up her coffee cup, she notices that both Sebastians jump up to tell him that he has the wrong cup in his hands. Alicia's face turns toward the poisoned cup and back to the faces of the Sebastian conspirators. She abruptly rises and excuses herself to go to bed. In distorted, point-of-view shots, Alicia's vision becomes totally blurred and hallucinogenic as she walks toward the silhouetted, shadowy figures of Alex and his mother. She collapses at the foot of the stairs onto the checkboard floor and is helplessly dragged upstairs and confined/imprisoned in her room to await the silence of death - Sebastian orders that the phone be disconnected and removed: "Disconnect the telephone. Mme. must have absolute quiet."

After waiting alone on the park bench and failing to rendezvous with Alicia after five days, Devlin meets with Prescott in his hotel room - his superior lies on his bed eating crackers and spread. He tells Prescott that he is suspicious that something is amiss with Alicia: "She wasn't drunk, she was sick. Maybe that's why she hasn't shown up. She looked like the ragged end of nowhere." Although Prescott argues that she may be drinking again, Devlin insists on seeing Alicia with a "social call" at Sebastian's estate as "a friend of the family".

In a memorable sequence, Devlin attempts to rescue a lifeless Alicia at the moment of her greatest suffering and weakness when he pays a call on the Sebastians. The enigmatic male lead cannot give himself up to her love and only reveals his own emotional needs in this final scene when he saves her from imminent death (from poisoned coffee). At the door, Joseph the butler hesitates when Devlin asks: "Family home tonight?" Devlin is admitted into the mansion's entry hall after learning that Mrs. Sebastian has been "very ill and confined to her bed."

As he waits there for a message to be delivered to Mr. Sebastian in his study (where he is meeting with his fellow Nazi conspirators), Devlin sneaks upstairs alone where he finds the semi-conscious, incoherent, almost catatonic Alicia in bed. Moving close to her, he whispers in her ear, asking what is wrong. She tells him that she is being slowly poisoned [dying in the same manner as her father] - tortured and victimized by the Sebastians (who know everything). Convinced that he must save her and protectively rescue her, Devlin compassionately avows his love for her and admits his own pain and suffering over her marriage to Sebastian. He also apologizes for acting badly and behaving cruelly toward her, and for denying his emotional vulnerability. The scene is a re-enactment of the Rio balcony scene as they openly reveal their love for the first time:

Devlin: Alicia, Alicia, Alicia. What's wrong with you? (Wearily, she reaches out for his hand as he bends down to her face on the pillow and whispers.)
Alicia: I'm so glad you came.
Devlin: I had to. I couldn't stand anymore waiting and worrying about you. It wasn't a hang-over you had that day. You were sick then. What is it?
Alicia: Yes, I was sick. (She caresses his face)
Devlin: What's wrong with you, Alicia?
Alicia: Oh Dev.
Devlin: What is it, dear? What's wrong with you?
Alicia: They're poisoning me. I couldn't get away from them. I tried but I was too weak.
Devlin: How long?
Alicia: Since the party. Alex and his mother found out.
Devlin: Come on, try to sit up. Sit up. I've got to get you out of here. (He struggles to pull her up from the bed.)
Alicia: I thought you had gone.
Devlin: No. I had to see you once and speak my peace. I was getting out because I love you. I couldn't bear seeing you and him together. (They hug each other)
Alicia: Oh, you love me. Why didn't you tell me this before?
Devlin: I know. But I couldn't see straight or think straight. I was a fatheaded guy full of pain. It tore me up not having you.
Alicia: Oh you love me. You love me.
Devlin: Long ago. All the time. Since the beginning...Come on, try to sit up.
Alicia: Oh Dev. I'm afraid I-I can't make it because they gave me pills to sleep.
Devlin: Keep awake. Keep talking...
Alicia: They didn't want the others to know about me.
Devlin: Keep talking. Come on, what happened? What happened?
Alicia: Alex found out.
Devlin: And the others haven't?
Alicia: They'd kill him if they knew. They killed Emil.
Devlin: Are you in pain?
Alicia: I don't know - the pills...(asking him to keep her alive): Say it again, it keeps me awake.
Devlin: I love you. Stand up. Stand up. Come on, wake up. Talk!

Draping her in her robe and mink coat, Devlin decides to pick her up and carry her lifeless, near-death body toward the front door to safety. She tells him that they killed Emil and that she knows where the "sand" comes from. Alicia is fearful that they will be discovered by everyone in the house and they won't be able to escape. Dazed as she leans on his shoulder, he takes her out of the second floor bedroom, struggling to keep her alive and talking. She begs: "Don't ever leave me." He vows: "You'll never get rid of me again." She asserts: "I never tried to." At the top of the stairs, they are discovered by Alex and Mme. Sebastian.

Devlin lets Sebastian know what he is doing with Alicia in his arms. He whispers what he knows to them, taunting them and bribing them to keep quiet. He warns Sebastian to not raise an alarm. He promises Sebastian that if he is allowed to get Alicia to a hospital, he won't disclose Alex's mistake about helping the American government to his colleagues:

I'm taking her to the hospital to get the poison out of her...How'd you like your friends downstairs to know? They've yet to be told...You haven't forgotten what they did to Emil, have you Sebastian?...You've got your chance [to die] here and now. Tell them [the Nazis] who she is.

As Devlin slowly descends the grand staircase toward the front door, half-carrying Alicia in his protective arms, the camera cuts back and forth from their slow movement to reaction shots of Alex, his mother, and the other Nazis downstairs. The Sebastians are rendered powerless to stop their departure. Humiliated and scared that his Nazi compatriots will be alerted to the fact that his wife is an American agent, Sebastian pretends to help Devlin and Alicia out of the house. To save himself from exposure to his Nazi friends, he tells them that Alicia must be rushed immediately to the hospital: "She collapsed. Mr. Devlin heard her scream when he was waiting for me." Devlin inadvertently condemns Alex with one contradictory detail: "Yes, I telephoned the hospital as soon as I saw how she was."

In the final memorable sequence, Devlin carries Alicia outside to his car - the open door behind them illuminates Alex's suspicious fellow-conspirators in the entryway. Alex follows them to the car to complete the masquerade. When he has Alicia and himself in the car, Devlin locks the car door and responds: "No room Sebastian." Alex pleads to escape with them to the hospital: "But you must take me, they're watching me." Devlin knows what Alex's fate will be when he returns to the house. He tells him as they prepare to drive off: "That's your headache."

In the entryway, one of the Nazis speaks out of the side of his mouth: "There is no telephone in her room to call the hospital." In the final image, Alex turns from the car and hears Eric Mathis, a renegade Nazi leader and one of his suspicious superiors, call him into the front door for an accounting [meaning his death]. He realizes that Alex has betrayed his confidence. With the film's last words, Eric requests:

Alex, will you come in, please? I wish to talk to you.

The door closes behind Sebastian as he enters, eliminating the light. Devlin has successfully liquidated an enemy spy as well as Alicia's husband - his rival to Alicia's love.

The film abruptly ends without a resolution to the couple's romantic union, or even without revealing whether Alicia's life will be saved at the hospital.