When the wind shifts to the South, cannons can be heard being fired in the city: "It's a fever cannon down in the city...Yes, at night, they set tar barrels blazin' at every street corner. And the Washington artillery fire cannon to fight the plague....Everybody knows it starts air currents to carry the fever away." Preston recommends the Yankee practice of draining swamps near the city, antagonizing himself toward Buck Cantrell's Southern persuasions:

Preston: They might better drain the swamps and clean up the city.
Buck: Is that what they do in Yankeeland?
Preston: They do. And a lot of other customs up North that'll bear copyin'.

Biding goodbye to Amy for a short while, Preston is summoned to ride by horseback to the city to assist Dr. Livingston with his patients during the raging yellow fever epidemic, one of whom is Mr. Jean Le Cour (John Litel): "He wants to talk some bank business to you before..., well, before it gets too late." Further insults and affronteries develop between Ted and Buck, as Preston's younger brother accuses Buck of being used and manipulated by Julie:

Amy: Oh, I do hope everything will be all right.
Julie: It always is. Of course it's a pity Pres had to leave now, but I suppose his bank comes first.
Amy: You don't find that admirable, Miss Marsden?
Julie: Oh, admirable, I suppose. It just seems to me there are other things more interestin'. I imagin' Buck finds it so.
Buck: Why, Miss Julie, I never go into a bank if I can help it. Seems like they're mostly studyin' how to get somethin' away from somebody.
Ted: I wouldn't say that, Buck.
Aunt Belle: (in a conciliatory tone) What Mr. Cantrell means is that most folks down here prefer other pursuits like plantin'.
Amy: You needn't apologize for Mr. Cantrell. I'm sure he's capable of defending his own insinuations.
Buck: Why, Mrs. Dillard, ma'am, I-I surely didn't mean to offend.
Ted: Of course, Amy, Buck didn't realize what he was sayin'. He doesn't even realize how Julie's usin' him.
Buck: How do you mean that, son? I just don't get the sense of it, but it don't sound polite.
Ted: You know right well what I mean. Julie's been eggin' you on, first against Pres and now his wife.
Julie: Ted, be quiet.
Ted: I meant what I said.
Buck: Well, let's figure I didn't hear it, then. And boy, I'd keep Miss Julie's name out of your conversation.
Ted: You're fool enough to defend what you don't even understand.
Julie: Buck, don't you pay any attention to him.
Buck: I-I don't see anybody that needs any defense. If I did, I'd make out to defend him. Yes.
Ted: (stepping forward) Let's put it this way, then, Mr. Cantrell. All evening long, I thought you coarse and loud and not a gentleman.
Buck: That's better. (He restrains Ted as he prepares a slap across the face) No need for that. Very well, Mr. Dillard, sir.

Buck Cantrell, a young gallant who eagerly abides by the Southern code of honor to defend against Ted's slight, challenges Preston's brother to a duel, even though Julie, who has actually engineered and instigated the conflict, forbids both of them. Julie's plan to win back Preston go "astray" and backfire, putting Ted's life in mortal danger:

Julie: Buck, I forbid you as a guest in my house.
Buck: Forbid me what, ma'am?
Julie: To do what you're plannin'.
Buck: I don't know what you mean, Miss Julie?
Julie: Stop play actin'! Your stupid code - it's for fools.
Buck: You too beginnin' to think our customs are old-fashioned?
Julie: Buck, I beg you. This mustn't happen.
Buck: I don't understand. I guess there's a lot I don't understand.
Julie: (to Ted) Ted, naturally, you will apologize to Buck.
Ted: General Bogardus, may I put myself in your hands, sir? You'll be good enough to make the arrangements.
Julie: Ted, listen to me, you can't do this. You must...(She realizes her appeal will be in vain) (To General Bogardus) Uncle B., you won't let this go any further, will you?
General Bogardus: Mighty little I can do about it now. Naturally, I'll do everything I can.
Julie: Whatever it is, you must do it.
General Bogardus: (contemptuously) Isn't this what you wanted, Julie? Or did your plans go astray?
Aunt Belle: We women can start the men quarreling often enough, we can't ever stop 'em.

The outcome of the duel between Buck and Ted is prolonged - the two fire shots at each other from outside the frame of the film. Ultimately, it is revealed that Buck is killed. To Julie, who has blood on her hands and is ultimately responsible for his death, Ted relates how Buck finally admitted knowing that he had been used: "I never saw a man die before. He knew what you had done. Before he died, he told me so."

General Bogardus reviles Julie, refusing to continue as her guardian: "I'll arrange to turn my guardianship over to the bank." And Aunt Belle, a model of civility, graciousness, refinement and gentility, also accuses Julie of being a Jezebel - bestowing the film's moniker on her. [Note: The Biblical Jezebel was the wife of King Ahab - her scheming caused a man to be killed.]:

Julie: Well say it. What are you thinkin'?
Aunt Belle: I'm thinkin' of a woman called Jezebel who did evil in the sight of God.

They are interrupted by the sounds of gunshots and barking bloodhounds. Outside, "a poor fool who was crazy enough to think he could run the fever line" is shot down as he attempts to pass the parish boundaries - "comin' or goin'." The Yellow Jack (yellow fever) epidemic is "spreadin' like wildfire all along the river..." No one is allowed to "go down river to New Orleans" - the assembled company is quarantined at Halcyon. Amy, General Bogardus, Mrs. Kendrick and Ted are uneasy as they are restricted from leaving the plantation by the sheriff. Even though she is held contemptuously by everyone, hostess Julie welcomes everyone back: "Ladies and gentlemen, my home is yours, as always."

Superimposed jagged letters zoom out from the screen: "Yellow Jack" - with jarring orchestration. Panic strikes the city of New Orleans in 1853. Martial law has been declared, and troops line the city streets to enforce the law. Burning barrels are placed on street corners to dispel the fever-vapors of the swamp air. Open wagons carrying the sheet-covered bodies of the sick and dead are transported through town to the docks, where the fever victims will be dispatched in boats to Lazarette Island. Cannons are fired to stir up the air currents to carry the fever away. Tempers flare among those who find sustenance in drink at the St. Louis Hotel. Accompanied by Dr. Livingston at the bar, Preston looks feverish and is feeling "kinda edgy maybe. It's been a tough day. I caught chill walkin' up." He is further agitated when he overhears that his brother killed Buck Cantrell in a duel - he faints onto the floor, and bystanders back away, yelling that he is stricken with the fever: "Yellow Jack!"

During a subdued, awkward dinner scene at the plantation, word is received from Dr. Livingston through black servant Bat (Eddie Anderson) that "Pres - he got the fever bad." Preston is being cared for in the Dillard's house "because the hotel folks won't take him in. He say you all get there right away quick before they hustle him off to that leper place." Julie is determined to return to Preston's side to nurse him to health, and she insists on leaving immediately and breaking through the blockade:

Julie: You're goin' back right now, and I'm goin' with ya. Zette, get me a cloak, I'll meet ya at the smokehouse.
Bat: Oh Miss Julie, Miss Julie, Miss Julie ma'am. It's too risky for a white lady, Miss Julie. Me, I kinda mix in with the night. But them sheriff folks ain't foolin'. They're shootin' first and askin' afters.
Julie: Bat - you hear me, we're goin'.

They bravely travel in an open rowboat through the misty swamps under cover of night, arriving in time before Preston is exiled to Lazarette Island. Cared for in her own upstairs bedroom because the hotel turned him away, Preston rolls around delirious. "Naturally, his place is here," she remarks devotedly. Dr. Livingston warns about catching the contagious fever: "You oughtn't to be in here, Miss Julie. The yellow jack's mighty catchin'...Just keep on applyin' the cold compresses. It's all we can do." Day and night, an exhausted Julie looks after the only man she ever loved - nursing him, and fanning and wetting down his sweaty face and forehead.

Having received a pass from the governor, Amy, Ted, and General Bogardus finally return to New Orleans, where they discover the house has been marked with big black Y's. Pres' condition is reported to be "no better, no worse." Julie is displaced from the bedside by Amy's arrival. Because Pres is a fever patient, he has been reported to the authorities by a dutiful Dr. Livingston:

Have you any idea what would happen in New Orleans now if folks got to thinkin' there was one law for the rich and another for the poor?...Pres is beyond talkin' for himself but I know him and I know his breedin'. I know what he'd say. He's a man of honor.

When the kindly Dr. Livingston advises Julie to get some sleep - "otherwise we'll be sendin' you off to the island too" - she sits upright, sensing that she might travel with Preston. Amy begs Dr. Livingston for an "unthinkable...terrible" favor - "You must arrange it so that I can go with my husband." However, Julie convincingly pleads her case to Amy, appealing to her in a moving and dramatic speech. Quickly and nobly transformed into a sacrificial Florence Nightingale, she begs her to be allowed to accompany her beloved husband because she is stronger and better suited to nurse him back to health:

Julie: Amy, of course it's your right to go. You're his wife. But are you fit to go? Lovin' him isn't enough. If you gave him all your strength, would it be enough?
Amy: I'll make him live or die with him.
Julie: Amy, Amy, do you know the Creole word for fever powder, for food and water? How to talk to a solid, over-worked black boy and make him feel he will help you? Pres' life and yours will hang on things just like that, and you'll both surely die.
Amy: Then it will have to be that way.
Julie: It's not a question of provin' your love by layin' down your life for Pres. Nothin's so easy. Have you the knowledge and the human strength to fight for his life and for your own as one will have to fight? Amy, it's no longer you or me.
Amy: What do you mean?
Julie: I'll make him live, I will. Whatever you might do, I can do more, 'cause I know how to fight better than you. Amy, if you knew the horror of that place. It isn't a hospital! It's a desolate island haunted by death. They'll put Pres in an open shed with a hundred others. You must be there with him day and night, watchin' every breath he draws. You must bathe him, keep him clean, give him drugs, fight for his food and water. You must keep the livin' from him and the dead. Be there by him with your body between him and Death.

The militia announces its arrival with loud pounding on the front door - they have come to evacuate Pres to the quarantine island. The men move up the stairs, passing the two ladies - their dark figures cross in front of the camera as they finish their conversation. Julie promises to care for Amy's husband - supplanting Amy's obligations as the wife. Humbled and contrite, Julie comes to terms with her own faults and begs for a chance to redeem herself from being a Jezebel, to prove herself worthy of her love for Pres ("brave and strong and unselfish"), and to "make myself clean again" with her ultimate sacrifice for him:

Amy: I'm not afraid.
Julie: No, you're not afraid. You're the bravest women I ever saw. I even believe you have the courage to save him by giving me the right to go in your place. I'm not afraid to die. I boldly ask a greater sacrifice in Pres' name - his life.
Amy: And for yourself?
Julie: I'm askin' for the chance to prove I can be brave and strong and unselfish. Help me, Amy. Help me make myself clean again as you are clean. Let me prove myself worthy of the love I bear him.
Amy: Julie, tell me, something that only you can tell me. Does Pres still love you? He himself might not know, but you would.
Julie: Amy, you must let me go with him.
Amy: Tell me.
Julie: What does it matter who he loves? It's his life that matters!
Amy: Tell me.
Julie: We both know. Pres loves his wife. Who else would he love? Not me, surely. I've done too much against him. You see, I never know how to be gentle and brave as you are. Had there been any love in his heart for me, I'd have taken him from you. I tried and failed 'cause he loves only you.
Amy: I'm grateful to you for telling me, but I had to know.

As Preston's body is borne down the stairs on a stretcher, Amy permits Julie to accompany her stricken husband, with her best wishes: "God protect you and Pres." Julie truly redeems herself by journeying to the hellish, condemned Lazarette Island (where the victims are incarcerated), assuring herself almost certain death, although she will be comforted by accompanying her beloved Pres. During a bumpy wagon ride bound for the wharf, Julie stoically cradles Preston's head in her lap. Flames from a burning barrel engulf the screen, and cannon-shots sound.

The film ends on this suspended scene, although the audience has to assume that no one will come back alive from the bayou island.