An unnamed narrator—a literary biographer and scholar who is devoted to investigating the life and work of deceased American Romantic poet Jeffrey Aspern—travels to Venice in order to try and obtain letters exchanged between Aspern and his ex-lover, the recluse Juliana Bordereau. He shares his plan with a fellow expatriate American living in Venice, Mrs. Prest, and tells her that he intends on renting a few rooms from Juliana and potentially becoming romantically involved with her niece, Miss Tita, to get the letters.
When he arrives at the Bordereaus' palazzo, the narrator is stunned by its grand size and its state of decay. Although it is immense, the mansion is almost entirely empty. The two women—Juliana and Miss Tita—have evidently lived as social hermits for the better part of the preceding decades, despite formerly being integrated into Venetian artistic and bohemian society. The narrator concocts a false story about his desire to remodel the gardens in the palazzo and uses a fake name to rent a few rooms from Juliana, who realizes that she can use the money to provide an inheritance for her niece. The narrator unsuccessfully tries to get Miss Tita to help him obtain the letters, but she confesses to not knowing if they even exist. The narrator realizes that because Miss Tita has lived most of her life in isolation, he can manipulate her into helping him by taking advantage of her naiveté and loneliness. This proves difficult, however, as Miss Tita is intensely devoted to her aunt, and fears betraying her by helping the narrator.
After a series of failed attempts to learn more about Juliana, the narrator is surprised when Juliana asks to see him—her advanced age means that she rarely emerges out of her room, but when he comes to see her, she is downstairs. She tells the narrator that he must pay for several more months' rent up front in order to continue staying at the mansion. The narrator hesitates at first, as he has almost no money left, but Juliana reveals that she knows why the narrator is at the mansion by showing him a portrait of Jeffrey Aspern and luring him into wanting to stay in order to find out more about the poet. Although the narrator denies knowing who Aspern is in an attempt to make Juliana less suspicious of him, he is unable to resist the intrigue of the portrait and the continuing mystery of the letters.
One night, after learning that Juliana is not in her room due to illness, the narrator sneaks into her room in order to look at the letters. Juliana walks in on him as he is reaching out to search the desk where he believes that the letters are. She screams and accuses him of being a "publishing scoundrel" before collapsing into Miss Tita's arms. The narrator, shocked and horrified, flees Venice for a few days. Upon his return, he learns that Juliana has died. He continues to try and persuade Miss Tita to give him the letters. Miss Tita hints that she will only give him the letters if he marries her; the narrator, extremely disgusted by her proposal, rejects her. However, he returns, hoping to renegotiate some kind of agreement with Miss Tita. She informs him that she has burned the letters and tells him to leave the mansion. As a parting gift, she gives him the portrait of Aspern that Juliana had originally shown him. The narrator returns to his home and is haunted for the rest of his life by the loss of the letters.
