The Bell Jar takes place during the early fifties and begins in New York City, during a sultry summer in which the narrator, Esther Greenwood, is an intern at a fashion magazine after winning a scholarship. She soon befriends Doreen, a fellow scholarship winner who is perpetually cynical and bemused. Doreen takes Esther out for drinks, where they meet several men, including Lenny Shepherd, a disc jockey. Esther and Doreen go back to Lenny's apartment, where Doreen and Lenny progressively become more intimate and even somewhat violent with each other. Esther flees the apartment to return to her hotel, where she only wishes to forget the experience that night. Later that night, Lenny brings the inebriated Doreen back to the hotel, where Esther cares for her despite a conviction that she will no longer associate with Doreen.
The next day, Esther attends a banquet for Ladies Day, the magazine where she works, and afterward her editor, Jay Cee, asks Esther what she plans to do after graduating from college. Esther is unsure, and Jay Cee reprimands her for such an indecisive attitude. Still, Jay Cee reassures Esther and tells her "don't let the wicked city get you down." After going to a movie with the other interns, all of them begin to feel sick and fall ill from food poisoning from the morning banquet. Only Doreen remains healthy, because she did not attend the banquet.
Mrs. Willard, the mother of Buddy Willard, a Yale student whom Esther had been seeing, arranges for Esther to meet with Constantin, an interpreter at the United Nations. Esther now hates Buddy for his hypocrisy and condescending attitude, particularly since he expects Esther to be pure despite his affair with a waitress during a previous summer. Buddy, who is now a student in medical schools, is currently in the Adirondacks recovering from TB, which Esther deems punishment for Buddy's double life.' While watching Constantin at work, Esther panics about her future, thinking that she knows nothing except how to win scholarships. She decides that she will let Constantin seduce her, but back at his apartment Esther and Constantin merely fall asleep beside one another. Esther reminisces about visiting Buddy at the sanatorium where he is recovering from TB, and how he proposed to her there. She refuses his proposal because she does not wish to be married, and admits that she is a neurotic.
During a photography session for the interns at Ladies Day, each of the interns must be photographed with props showing what they want to be, but Esther cannot choose. Jay Cee claims that Esther wants to be everything, and Esther finally decides to be a poet, but during the shoot Esther breaks down into tears. During Esther's final days in New York, Doreen sets up Esther with Marco, a Peruvian man who Esther decides is a woman-hater.' He attempts to rape her, but she fights him off.
Esther returns home to the suburbs of Boston, where her mother tells her that she did not make the writing course for which she applied and that she would be spending the rest of the summer there. She considers staying with friends at Harvard despite not getting into the writing course there, but then vows to read Finnegans Wake instead and to work on her thesis. Soon, however, she decides to not write a thesis and quit the honors program. Esther finds herself unable to sleep and goes to see her family doctor to get sleeping pills, but her doctor instead tells her to see a psychiatrist instead.
Esther tells the psychiatrist, Dr. Gordon, how she is neither sleeping nor eating, but Dr. Gordon only asks where Esther goes to college before telling her that he will see her next week. During the next session, Esther shows Dr. Gordon her handwriting (Esther has been unable to write), and he merely asks her if she minds if he spoke to her mother. Dr. Gordon advises shock treatments for Esther, who begins to think more and more often about suicide.
The hospital where Esther receives shock treatments is an alien place for Esther; the people there seem counterfeit and inanimate. Esther loathes the shock treatment, and tells her mother that she will not see Dr. Gordon and will not go back for treatment. Her mother merely responds that she knew that her baby wasn't like the awful people in the hospital. Soon after, Esther prepares a hot bath so that she can commit suicide through slitting her wrists, but she can only bear to make a practice' cut on her knee.
Esther considers killing herself by hanging, but her house has the wrong type of ceiling to do so, and attempts to drown herself while swimming with friends but cannot. Finally, Esther writes a note to her mother that she is going for a walk, but instead hides herself and takes a bottle of sleeping pills. She awakes in complete darkness and believes that she has gone blind. When she regains full consciousness, a doctor reassures her that her sight is perfectly intact. Esther receives numerous visitors as she recovers, but dislikes the attention she receives from them. She behaves truculently toward several of these visitors and the nurses, who tell her that she'll be taken care of at "you-know-where."
Nevertheless, Esther does not go to the state mental institution, but instead to a private facility thanks to the intervention of Philomena Guinea, the philanthropist who provides for Esther's scholarship and who learned over her suicide attempt through media coverage. At this facility, Esther stays at the Caplan wing and is treated by Dr. Nolan, a female psychiatrist who reminds Esther of a cross between her mother and Myrna Loy. Dr. Nolan promises not to administer shock treatment to Esther, but if she does to tell her in advance. Esther gets a surprise visitor at Caplan from Joan Giling, another old girlfriend of Buddy Willard who has checked into the hospital after attempting suicide herself.
After Esther has a reaction to the medicine Dr. Nolan has prescribed for her, Dr. Nolan becomes concerned that Esther is not making progress and prohibits her from having visitors. Esther reacts to this news by exclaiming that it's wonderful, for she hates the visits from Philomena Guinea, former acquaintances and in particular her mother. When Esther tells Dr. Nolan that she hates her mother, Dr. Nolan smiles as if Esther has said something pleasing.
Dr. Nolan moves Esther to Belsize, a different hospital ward that entails greater privileges. Joan is now in Belsize as well, where the patients are fashionably dressed and made-up. However, Esther finds one morning that Dr. Nolan has scheduled her for shock treatments and not told her (she believed that if she told Esther the night before, she would not sleep). Esther endures the shock treatment, and Dr. Nolan attempts to reassure Esther that it was not like it was before. After Joan and Esther both receive letters from Buddy Willard, Joan wonders whether he will visit the asylum. Joan tells Esther that she likes her more than she likes Buddy, but Esther, suspecting lesbianism, tells Joan that she makes her puke. Esther gets birth control pills, which she finds will be freedom from marrying the wrong person like Buddy Willard, but then vows to find a proper man.
Joan is released from the institution, and while Esther visits her in Cambridge, she meets a Harvard mathematics professor named Irwin. Esther decides to seduce him, but after she has sex she begins to bleed heavily. Joan takes Esther to the hospital, for she is hemorrhaging. Esther returns to the institution, and Joan even moves back there herself, but soon afterward a doctor awakes Esther to tell her that Joan is missing, and later that Joan has been found in the woods. She has hanged herself.
Esther prepares to leave the asylum that January when the next semester begins, but remains there until it is time to move back to the dormitory. She realizes that people will treat her differently, and her mother thinks about her time at the institution as a "bad dream." Buddy Willard visits Esther at the institution, and because of Joan and Esther wonders if there is something in him that drives women crazy. Esther responds to this by laughing. Esther wonders whether she will suffer the same depression again, but feels that she is now perfectly free. She leaves the hospital "patched, retreaded and approved for the road."