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In turn-of-the-century New Orleans, beautiful but impoverished aristocrat Barbara Beaurevelle vows to join her lover, medical researcher Dr.  Mark Lucas, that night on a South American-bound ship and marry him.  When her scheming cousin Paul finds out her plans, however, he reminds her about her aunt Eula's heart condition and the pain that such an elopement would cause her.  Paul persuades Barbara to write Mark a letter asking him to wait for her and promises to deliver it, but never does.

Two months later, on Halloween, Barbara eagerly awaits Mark's return at the docks and is stunned to discover that he has married a woman he met during his travels.  Confused and angry, Barbara defies her aunt and goes to see Luther Toplady, a lawyer who has been trying to locate her about an inheritance.  Luther informs Barbara that her grandmother Carrie Crandall's brother has died and left his fortune to her.  Because Carrie led an infamous life, Barbara's relation to her has been kept secret, and Barbara asks that Luther transfer the money to her without mention of Carrie's name.

Barbara then breaks an engagement with Clay Duchesne, her wealthy suitor, and goes alone to a cemetery, which is filled with Halloween revelers.  There, Barbara spies Mark as he and his mentor from Tulane University, Dean Cazzley, give Mark's bride Corinne a tour.  Still unaware that he never received her letter, Barbara pulls Mark aside and tries to rekindle his passion for her.  Mark gives in momentarily, kissing her, but refuses to betray Corinne any further.  Barbara then shocks a young candle seller by lighting a candle at her notorious grandmother's grave.

The next day, Barbara informs her aunt and Paul about her million-dollar windfall.  Aunt Eula, who dreads scandal, is distressed by the news but Paul is ecstatic.  Sometime later, the now-rich Barbara visits Corinne at home to invite her to a ball.  Barbara and Corinne exchange coy barbs, and Corinne reveals herself as a calculating social climber.  When Mark learns about the ball, he goes to see Barbara to turn down the invitation, but instead is insulted by the snobbish Paul.  Irked, Mark attends the ball with Corinne, who is immediately attracted to Paul.  Once alone with Mark, Barbara finally learns the truth about her letter and asks Mark to divorce Corinne and marry her.  Although Mark kisses Barbara again, he disdains her casual attitude toward marriage and storms off.

Thus scorned, Barbara plots to humiliate Mark by paying Paul to seduce Corinne.  Paul easily charms Corinne and arranges a boathouse tryst.  When Barbara informs Mark about Corinne's planned rendezvous with Paul, he quietly condemns her vengeful ways.  Suddenly filled with remorse and self-loathing, Barbara heads for the boathouse.  There, Corinne tries to convince Paul to marry her, and when he laughingly rejects her, she slaps him.  Angered, Paul pushes Corinne, causing her to fall and suffer a fatal head injury.  Barbara sees Paul run off and goes inside the boathouse to find Corinne.  Moments later, Mark arrives and, deducing what has happened, instructs Barbara to leave.  Mark immediately falls under suspicion, and the influential Clay, who has long been jealous of him, demands his dismissal from Tulane.

During the ensuing inquisition, Mark does not mention either Barbara or Paul and faces indictment for murder.  Unable to stay silent, as Paul and Eula have instructed her, Barbara reveals all to a stunned courtroom, including her connection to Carrie.  Although Eula disowns her, Mark applauds Barbara's courage, noting that she could turn out to be "quite a woman."