The Old Guard of 1852 New Orleans protectively circles its wagons. Bankers are concerned that Yankee abolitionists could eventually change the way their accounts will be processed. The health administrator keeps evacuating people living below the ever-adjusted yellow fever quarantine line.When Julie (Bette Davis) has to move up from her own mansion below the fever line to become a guest in her Aunt Belle (Fay Bainter)'s mansion, her interested beaux include Buck Cantrell (George Brent), a southern dandy rake, and Preston "Pres" Dillard (Henry Fonda), the most wooden of the old guard bankers who is on the committee to arrange that annual ball which soothes the senses of folks looking to maintain the old ways. Pres is so engaged in helping his bank react appropriately to perceived abolitionist Yankee threats that he is unable to drop by the dressmakers to approve the ball gown Julie is commissioning. Unaware of the purpose of the ball, and unaware that red is the biblical rainment of the fallen woman, - all the rest of the ladies at the ball will be wearing white frogs - Julie spitefully commissions a rather revealing red gown. Aunt Belle and the health administrator beg Pres to beat some sense into her. When Pres, who is, in his own way, as socially unaware of the world as is Julie, storms into Julie's room brandishing a cane, Julie jokingly interprets his behavior as she would that of any powerful, athletic man forcing himself into a lady's private bedroom brandishing a weapon. Pres decides then and there to rethink his assessment of Julie as his proper wife. At the ball, Julie is so flabbergasted to be shunned that she begs Pres to take her home. Pres, however, decides that it would be best to publicly announce their breakup in the presence of all of New Orleans Society. After Julie is humiliated into dancing with Pres, the other couples move off the floor and the orchestra is ordered to stop playing, leaving the couple isolated on the dance floor with everyone facing away from Julie.Some time later, Aunt Belle gives a ball to welcome Pres returning home from banking business up north to present Amy (Margaret Lindsay), his aristocratic Yankee wife. Julie dresses up in a white party gown which shows a lot of cleavage and could be that of a wedding. She bosses Zette (Theresa Harris) onto checking whether somebody has arrived, dressing her, going after her... Finally, Pres arrives in a carriage. After Julie's carefully rehearsed, staged and white-dress costumed act of contritely begging Pres for forgiveness is considered by Amy to be totally inappropriate, Julie makes a second faux pas in the presence of all New Orleans Society by publicly both seducing Pres and insulting Amy. Because Pres acquired no social skills at all while up north, he decides that now is the time to inform Julie that she and Amy will both be guests of Aunt Belle while he answers to call to help the health administrator readjust the yellow fever quarantine line. There is a discusion between several men as to the importance of cotton in the country's economy, and they bow to keep New Orleans as the biggest city in USA. The rest of the gathering is outraged when Pres predicts that new machinery will beat unskilled slave labour. Buck and Julie seem to mock Pres, saying that he's talking like a Yankee. Pres has succeeded as a banker, and has lost Southern routines. During the night's small gathering at their manor house, Julie asks Pres why he's married so fast, and he replies back that he's in love with Amy. Julie says that he must have needed to come back because, after all, he belongs to the South.Buck mocks Amy in front of Julie, but she is too upset about Pres' response. However, Buck comments about Cato make him look foolish and redneck. Julie makes Buck promise that he won't start any fight with Pres. Cannonball thunder is heard: it's the cannonballs which are being thrown in order to change the winds which are spreading the yellow fever. There is a small discussion about how the yellow fever could be better stopped, and Pres says he would advise the city hall to drain the moors, clean up the street and organize a better sewage system. Buck, General Bogardus (Henry O'Neill) and Belle disagree: Southern traditions are good enough for them, and they must be kept. At that moment, the black slaves appear to sing welcoming songs to the guests. Julie sits down on the porch to sing with them.Buck is soon killed by Pres' younger brother Ted (Richard Cromwell) in a duel over Julie. Julie had started it all, encouraging once against the other. Dr Livingston (Donald Crisp) tells Julie that now she's got what she's been fighting for. Julie tries to convince him to stop the duel, but he says that women can start fights, but that they can't put a stop to them afterwards. Later, Julie is organizing the buquets of flowers onto their vases with Zette like she's got no a care in the world, while Amy gets frantic, taking for granted that Ted will die in the duel. However, Ted arrives saying that Buck die knowing that Julie had used him. Aunt Belle warns Julie that the community's mutual sufferings because of her outrageous behavior has the town comparing her to the biblical Jezebel.The fever spreads. New Orleans has been sieged: nobody can enter or leave the town. Pres drinks alcohol - the only known healing to the yellow fever. Banks are also suffering, as the plague is causing communication problems, the blockade, commerce problems... Pres says that he's feeling bad to Dr Livingston. After knowing that Buck is dead, Pres has a fit. Only Dr Livingston dares to help poor Preston Dilliard, who has crumbled down. When news arrives that Pres has contracted yellow fever, Amy confronts Julie to take stock of what she's done. Finally admitting to herself that Pres hasn't given a damn about her since the day of the ball, Julie realizes that her possible redemption lies not with those who will always shun her, but with the home that she really owns and the man she really wants each trapped below the quarantine line. A slave brings the news that Pres is at Julie's home because no hotel wanted to accept Pres. The slave was able to cross the quarantine line by means of a stolen boat. While the rest of her and Pres' relatives search for a governor's permit to visit Pres, Julie uses the stolen boat. Julie arrives first. Livingston advises Julie not to see Pres, as the yellow fever is really contagious. Pres is delirious. Julie tends to him. Belle, Amy and Ted appear with a Governor's pass to visit Pres. Julie is there. Amy looks at her with jealousy and amazement, and in silence she takes Julie's place besides the sick man's bed. Julie leaves in silence as well, and goes down the stairs, sitting carelessly in a sofa. Livingston says that Julie should sleep some, or she'll have to join Pres. That gives an idea to Julie.Ted is indignant that Livingstone has denounced Pres, so he'll be taken away to the leprosery, but the doctor says that there cannot be a law for poor people and other for wealthy people. The ruthless army people arrive. So Julie convinces Amy that she will accompany Pres to the leprosery to the island where he'll be sent, as only Julie will be able to take care of him, terrorising the nurses and workers onto giving him the best medicines and care. Amy refuses af first, but then she accepts, when Julie admits that Pres only loves Amy, and not her. That way, Julie will pay her dues and make atonement for her sins.Everybody stares while Julie goes running after Pres. They are two more people among the sick destitute who will go to the leprosery, most of them to die. While this happens, cannonballs fly and there are bonfires in every crossroad of the city.