A mystery man tries to help a young innocent escape a murderous housekeeper.

In October 1848, when young Frenchwoman Madeline Minot arrives at the New York townhouse of Thevenet, the elderly grandfather of her lover, Paul De Lage, she is disturbed to observe a rowdy party inside. Thinking that she must have the wrong address, Madeline goes to nearby Flaherty's tavern and confides in Dupin, a kind, but cynical man who is unable to pay his bar bill. Although Dupin advises her to go back to France, she returns to the house with a letter from Paul, a French patriot who has been estranged from his grandfather for many years. Once admitted to the house, Madeline is suspicious of the glamorous Lorna Bounty, Thevenet's housekeeper, and frightened of Thevenet's pet raven, Villon. Thevenet, who is dying of dissipation, dismisses her letter, which seeks money to finance the intellectuals of the new French revolution, but allows her to stay. Later, in the kitchen, Lorna, Martin, the butler, and Mrs. Flynn, the cook, express concern that after years of waiting for Thevenet's money, they may be cheated out of it by Madeline. That night, as Madeline sleeps, Thevenet enters her room and warns her to be careful. The next morning, when Madeline goes to mail a letter to Paul, she sees Dupin entering the tavern and asks for his help, telling him that the servants are trying to poison Thevenet. Dupin accompanies her to a pharmacist, who tells them that the substance she found in Thevenet's room is merely sugar water. Dupin advises her to see Thevenet's attorney, Durand, deducing that the servants are withholding the old man's medicine. Dupin then goes to the house on the pretext of looking for Madeline and meets Lorna, whom he recognizes as a former stage actress. That night, as a wild Halloween party is being held at Thevenet's, Madeline again goes to Dupin, worried that Lorna is killing Thevenet with drink. Dupin then goes to the house and flirts with Lorna until Thevenet suddenly screams that everyone should get out and asks to speak with Dupin alone. Thevenet, who likes Dupin because he is both a cynic and a poet, tells him that Madeline needs a friend. After Dupin warns Thevenet that he is a fool to reward his murderers with his fortune, Thevenet orders Martin to send for Durand. Later, Dupin accepts a loan from Lorna to pay his rent and tavern bill, then kisses her. After he leaves, Martin follows him, but Dupin eludes him. The next evening, as Dupin dines with Madeline in a restaurant, Thevenet dictates a new will to Durand, then secretly drops poison into his bedside brandy glass. Just after signing the will, he starts to have an attack, and an unnerved Durand finishes the glass of brandy and calls for the doctor. When Madeline and Dupin return to the house and see a coffin being carried out they assume that it contains Thevenet's body but learn from Mrs. Flynn that it is Durand who has died. They go to see Thevenet, who can neither speak or move, apparently the victim of a stroke. Thevenet uses the movement of his eyes to draw Dupin's attention to the empty brandy glass and further glances toward Villon and the barometer on the wall. Unable to decipher Thevenet's clues and suspicious of Lorna's lack of concern, Dupin takes the glass and leaves. Madeline runs after him and Dupin says that he had been concerned that Lorna might kill Durand if Thevenet wrote a new will and tells her about Thevenet's clues. At the pharmacist's, they learn that the glass contained arsenic that Thevenet himself had bought many months before. Dupin then deduces that Thevenet intended to commit suicide and Durand's death was an accident. Dupin and Madeline return to the house just as the doctor pronounces Thevenet dead. As Martin searches for the will, Lorna tells him to pack Madeline's things, then orders her out of the house. The next day, at Flaherty's, Lorna approaches Dupin and feigns sorrow over Thevenet's death, then offers him more money if he will help her locate the new will. He refuses the money, but agrees to help her find the will, then mysteriously suggests to Flaherty that he might never come back. At the house, Dupin tells Lorna that the new will is in the bedroom, then suddenly realizes that Villon must have put the will in the fireplace beneath the barometer. Dupin grabs the will and tries to run, but Martin fights him. When the local policeman, who was alerted by Flaherty, arrives, Dupin reads aloud Thevenet's will: all of the money goes to Paul, but the house and furnishings are bequeathed to Lorna and Martin, provided that they never move or sell anything from the house, so that they can live out their days in "comfort and happiness." As Dupin walks away, Lorna asks for her money and he says that someday he will not be hard to find. At the tavern, Madeline talks with Flaherty, who shows her Dupin's IOU, which reads "IOU $8.70--Edgar Allan Poe." Flaherty concludes that it will never be worth anything.