Steppenwolf begins with a preface narrated by the nephew. The nephew meets Harry Haller, or Steppenwolf, when he rents rooms at the aunt's boarding house. The nephew dislikes Steppenwolf immediately because he seems shy, unsociable, and snobbish. The nephew admits that he grows to like Steppenwolf after spending time with him and learning about his divided nature. He views Steppenwolf as "a genius of suffering." The nephew tells the reader that he will present Steppenwolf and his story without any commentary, but he repeatedly interjects "psychological observation[s]." Steppenwolf leaves his manuscript to the nephew, who then publishes it. The text, or "Harry Haller's Records," is the result.
The remaining chapters, except for the treatise section, are narrated by Steppenwolf. Steppenwolf explains his disgust of bourgeois society, as well as his fascination with it. He chooses to live among the bourgeoisie even though he expresses contempt for them. This is demonstrated when he rents rooms from the aunt, just as he has rented rooms from countless other bourgeois individuals in the past.
Steppenwolf describes a typical day for himself. Everyday is the same for him now that he is an "old" man in his fifties. He reads the paper, reads books, checks the mail, and loses himself in the humdrum routine. His discontentment with society, his dissatisfaction with his own life, and his conviction that he has a divided nature, compel him to consider suicide. Later, Steppenwolf sees a sign: "MAGIC THEATER. ENTRANCE NOT FOR EVERYBODY. FOR MADMEN ONLY!" A peddler gives Steppenwolf a pamphlet entitled, "Treatise on the Steppenwolf."
The point of view changes to third person within the treatise. The treatise seems to describe Harry Haller/Steppenwolf perfectly, even down to the names. According to the treatise, Steppenwolf possesses two natures — the human and the wolf. The human desires order, respect, and responsibility — all elements associated with the bourgeoisie. The wolf, on the other hand, only desires physical pleasure and physical gratification. It doesn't matter how he gets it. Steppenwolf's dualistic nature results in constant turmoil since each nature vies for control. As a result, Steppenwolf rarely has a moment of peace, and he feels isolated from others.
The treatise goes on to explain that bourgeois society flourishes as a result of all the Steppenwolves. The fact that there are many Steppenwolves is a given since every individual consists of multiple selves. Because society does not accept or understand the concept of multiplicity within the self, all Steppenwolves are destined to be isolated, rejected, misunderstood, and suicidal. The treatise states that suicide is not the answer.
Steppenwolf compares the self-portrait provided by the treatise to a self-portrait he has already written in the form of a poem. After contemplating both descriptions of himself, he decides to commit suicide. Later, he runs into an old acquaintance, the professor, who invites him to dinner. Steppenwolf accepts the invitation but has a sense of foreboding.
At the professor's house, he sees a picture of Goethe that repulses him. The professor mentions a newspaper article about a traitor with the name Harry Haller. Steppenwolf criticizes the picture of Goethe, admits that he has lied to the professor, and informs the professor that the newspaper article is, indeed, about him. He leaves and decides to commit suicide when he arrives home. He realizes he is afraid of death and even more afraid to kill himself.
He visits the Black Eagle and meets a young woman sitting at a table. She orders him to eat, drink, and sleep. Steppenwolf falls asleep and dreams of Goethe, a black scorpion, Mozart, and a woman's leg in a box. Goethe tells Steppenwolf that he is taking life too seriously. Steppenwolf awakes, and the young woman returns to tell Steppenwolf she is leaving with someone else. He asks her to dinner, and she accepts. She mentions that she understands his reaction to Goethe's picture because she does not like artists' renditions of the saints.
Steppenwolf meets the young woman again, and he guesses her name is Hermine. Hermine informs Steppenwolf that he will fall in love with her, and she will command him to kill her. They discuss the treatise, a newspaper article calling him a traitor, and the possibility of another war. He purchases a gramophone, and she teaches him the fox-trot. He sees Maria for the first time and dances with her.
Hermine praises Steppenwolf's newly acquired dance skills, and she informs him that he needs to have sex with a young woman. He protests due to his age, but she dismisses him. He discusses music with Pablo, the musician. Later, Steppenwolf attends a recital. When he returns to his rooms, he finds Maria waiting in bed for him. They begin a sexual relationship that lasts for the three weeks leading up to the Fancy Dress Ball. Steppenwolf is disappointed that he is not Maria's only lover, but he is grateful for her attention, and he is grateful to Hermine for arranging the affair.
Pablo invites Steppenwolf and Maria to participate in an orgy, but Steppenwolf refuses. Pablo admits that he and Maria are lovers too. Steppenwolf discovers that Hermine and Maria discuss their sexual experiences with one another. Steppenwolf even suspects that Hermine and Maria could be lovers. Hermine reminds Steppenwolf that they will be lovers at a later date.
Hermine explains that she is a prostitute only because her life fell short of her expectations, just as Steppenwolf's did. She had envisioned great things for herself, but as life turned out, she ended up a courtesan. Steppenwolf admits that he is very happy with Maria, but he believes the affair is only temporary. He knows that death awaits him. Hermine agrees and admits she is waiting for death and eternity as well.
Steppenwolf stops in a theater playing The Ten Commandments while on the way to the ball. He arrives at the ball and wanders around looking for Hermine. He is just about to leave when he is given a ticket with the following scrawled on it: "TONIGHT AT THE MAGIC THEATER. FOR MADMEN ONLY. PRICE OF ADMITTANCE YOUR MIND. NOT FOR EVERYBODY. HERMINE IS IN HELL."
Steppenwolf finds Hermine dressed as his old childhood friend Herman. They each dance with other women until Steppenwolf falls in love with Hermine. Hermine leaves and returns dressed as a dancer. The ball ends, and Pablo invites Steppenwolf and Hermine into the Magic Theater. Steppenwolf visits several galleries in which he relives and edits his past, observes his wolf and human natures in contention, murders people in a violent battle, and kills Hermine.
Mozart appears and chastises Steppenwolf for his lack of vision and elitist attitude. Mozart informs Steppenwolf that he is no different from the bourgeois society he detests, and he explains the Steppenwolf is responsible for his own discretions, as well as the faults associated with the time period in which he lives. Pablo appears, and Steppenwolf learns that he did not really murder Hermine.