The Black Prince tells the story of Bradley Pearson, a fifty-eight year old man who has previously published three books. In order to write a great novel, he quits his lifelong job as a tax inspector, but soon finds himself struck with writer's block. He decides to spend the summer in a rented cottage on the coast for inspiration.
Before he can leave for the coast, however, a series of events keeps him home. When his detested ex-wife's brother, Francis visits him he finds out that his ex wife, Christian, has returned to London. He is called to intervene in a marital dispute between his close friends, Arnold and Rachel Baffin. Arnold is a successful but unartistic writer. During the fight, Rachel Baffin hits her head on the fireplace poker, but is not dead, as Arnold initially fears. After leaving their house, Bradley runs into the Baffins' twenty-year-old daughter, Julian, by the subway station. She wants Bradley to teach her how to write. The next morning, Bradley's sister, Priscilla, unexpectedly arrives, because she has left her husband. She almost immediately tries to commit suicide with sleeping pills. During the confusion of her suicide attempt, for which all of the Baffins and Francis Marloe are present, Christian, Bradley's ex-wife, appears, but is taken away by Arnold before Bradley sees her.
After Priscilla gets back from the hospital, Bradley visits Christian in order to tell her to leave him alone. Bradley then goes to Bristol to pick up his sister's jewels. He is unable to do so, and finds that Priscilla's husband has a younger, pregnant mistress. Priscilla starts staying at Christian's house so Francis Marloe, a former doctor, can care for her. While all of this is happening, Arnold Baffin becomes interested in having an affair with Christian and Rachel Baffin becomes interested in sleeping with Bradley. During Rachel and Bradley's attempted lovemaking, however, he cannot perform sexually. He and Rachel later determine to become platonic friends.
Julian has been pestering Bradley to teach her about Hamlet and arrives one day for a tutorial. During the tutorial, Bradley falls passionately in love with her. He initially tries to keep his love secret. After becoming physically ill while watching Der Rosenkavalier with Julian however, he confesses his emotions. He tells Julian that he is forty-six, instead of fifty-eight. Julian considers the issue of his love thoughtfully. By the next morning, she has determined that she loves him. Julian later confesses her love to her parents. They respond by locking her in her room and yelling at Bradley. Despite Rachel and Arnold's anger, Bradley refuses to see that his love is inappropriate. When Julian sneaks away from her parents' house, she and Bradley meet and leave for his rented cottage.
On their first day away, Julian entertains romantic fantasies about marrying Bradley. Their initial attempts at lovemaking are not successful. The next day, Bradley finds out that Priscilla has killed herself. He keeps the news from Julian to maintain their bliss. When he returns home, he finds Julian dressed up as Hamlet. He drags her to the bed and makes violent love to her in such a rough way that Julian later weeps. Arnold finds them later that night and begs Julian to leave. He tells her of Priscilla's death and Bradley's true age. Julian seems confused, but refuses go. After her father leaves, she isolates herself in a separate bedroom to think, but is gone by the time Bradley wakes in the morning.
Bradley goes back to London for Priscilla's funeral. He believes that Arnold stole Julian away in the night. Bradley cannot find her anywhere. Christian wants to start a relationship with Bradley, but he declines. Rachel tells Bradley that Julian left him freely because she learned of Bradley's recent sexual encounter with Rachel (Rachel had described the encounter in a letter that Arnold delivered). Bradley is so angry at Rachel's interference that he spitefully shows her a letter that Arnold wrote describing Arnold's love for Christian. Rachel is furious and vows never to forgive Bradley. A few days later, Bradley receives a letter from Julian. Despite her saying otherwise, he decides that she still loves him and that she is in Venice. He makes plans to go there. Before he can leave, however, Rachel calls and begs for his immediate assistance. After arriving at the Baffins' house, Bradley finds Arnold dead, having been hit with the same fire poker that once hit Rachel. When Bradley tries to cover up Rachel's crime, he is accused of it himself. He is later convicted because everyone believes that he killed Arnold out of envy. Bradley has written his novel from prison. In the final postscript of the book, the editor, P.Loxias, notes that soon after finishing the book, Bradley Pearson died of a fast growing cancer.