Mrs. Dalloway covers one day from morning to night in one woman’s life. Clarissa Dalloway, an upper-class housewife, walks through her London neighborhood to prepare for the party she will host that evening. When she returns from flower shopping, an old suitor and friend, Peter Walsh, drops by her house unexpectedly. The two have always judged each other harshly, and their meeting in the present intertwines with their thoughts of the past. Years earlier, Clarissa refused Peter’s marriage proposal, and Peter has never quite gotten over it. Peter asks Clarissa if she is happy with her husband, Richard, but before she can answer, her daughter, Elizabeth, enters the room. Peter leaves and goes to Regent’s Park. He thinks about Clarissa’s refusal, which still obsesses him.
The point of view then shifts to Septimus, a veteran of World War I who was injured in trench warfare and now suffers from shell shock. Septimus and his Italian wife, Lucrezia, pass time in Regent’s Park. They are waiting for Septimus’s appointment with Sir William Bradshaw, a celebrated psychiatrist. Before the war, Septimus was a budding young poet and lover of Shakespeare; when the war broke out, he enlisted immediately for romantic patriotic reasons. He became numb to the horrors of war and its aftermath: when his friend Evans died, he felt little sadness. Now Septimus sees nothing of worth in the England he fought for, and he has lost the desire to preserve either his society or himself. Suicidal, he believes his lack of feeling is a crime. Clearly Septimus’s experiences in the war have permanently scarred him, and he has serious mental problems. However, Sir William does not listen to what Septimus says and diagnoses “a lack of proportion.” Sir William plans to separate Septimus from Lucrezia and send him to a mental institution in the country.
Richard Dalloway eats lunch with Hugh Whitbread and Lady Bruton, members of high society. The men help Lady Bruton write a letter to the Times, London's largest newspaper. After lunch, Richard returns home to Clarissa with a large bunch of roses. He intends to tell her that he loves her but finds that he cannot, because it has been so long since he last said it. Clarissa considers the void that exists between people, even between husband and wife. Even though she values the privacy she is able to maintain in her marriage, considering it vital to the success of the relationship, at the same time she finds slightly disturbing the fact that Richard doesn’t know everything about her. Clarissa sees off Elizabeth and her history teacher, Miss Kilman, who are going shopping. The two older women despise one another passionately, each believing the other to be an oppressive force over Elizabeth. Meanwhile, Septimus and Lucrezia are in their apartment, enjoying a moment of happiness together before the men come to take Septimus to the asylum. One of Septimus’s doctors, Dr. Holmes, arrives, and Septimus fears the doctor will destroy his soul. In order to avoid this fate, he jumps from a window to his death.
Peter hears the ambulance go by to pick up Septimus’s body and marvels ironically at the level of London’s civilization. He goes to Clarissa’s party, where most of the novel’s major characters are assembled. Clarissa works hard to make her party a success but feels dissatisfied by her own role and acutely conscious of Peter’s critical eye. All the partygoers, but especially Peter and Sally Seton, have, to some degree, failed to accomplish the dreams of their youth. Though the social order is undoubtedly changing, Elizabeth and the members of her generation will probably repeat the errors of Clarissa’s generation. Sir William Bradshaw arrives late, and his wife explains that one of his patients, the young veteran (Septimus), has committed suicide. Clarissa retreats to the privacy of a small room to consider Septimus’s death. She understands that he was overwhelmed by life and that men like Sir William make life intolerable. She identifies with Septimus, admiring him for having taken the plunge and for not compromising his soul. She feels, with her comfortable position as a society hostess, responsible for his death. The party nears its close as guests begin to leave. Clarissa enters the room, and her presence fills Peter with a great excitement.