Mrs. Dalloway is a unique novel in that it takes place in a single day — a Wednesday in mid-June 1923. The novel interweaves two seemingly unconnected storylines during this day.
At the beginning, Clarissa Dalloway, fiftyish and recently recovering from an illness, is preparing for a party she will host that evening. She begins her day running an errand to purchase the flowers for the party. Throughout the morning, Clarissa reflects on her past, including her decision to marry Richard Dalloway thirty years earlier, rather than her more fiery suitor Peter Walsh.
Meanwhile, the second storyline begins with Septimus Smith, a shellshocked war veteran, out on the street with his wife, Lucrezia. Septimus struggles with the aftereffects of the war, hearing voices and feeling that life has little meaning. A car backfiring paralyzes him, and he reflects on his life. Septimus lost his good friend and commanding officer Evans in the war and continues to carry on conversations with this lost friend.
Clarissa has returned home and begins to remember a special friendship she shared in her youth with Sally Seton, a vivacious, slightly scandalous young woman. The two shared a special bond, bordering on a crush, and Clarissa remembers a kiss they shared.
Clarissa begins mending her green silk dress for the evening when she receives an unexpected visit from Peter Walsh, her former suitor. Peter had once told Clarissa disparagingly that one day she would become "the perfect hostess," and it becomes more and more clear that his prediction was accurate. Clarissa and Peter talk to each other easily about the present, but both are thinking of their past and the decisions they made to get them to the place they are now. Clarissa's 17-year-old daughter Elizabeth enters and Peter ends his visit.
Peter goes to a park where Septimus and Lecrezia are also walking. The couple get into a heated discussion about suicide, and Peter sees them as a young and in love couple quarreling. He doesn't realize the depth of their emotions or how unsteady Septimus is. Lecrezia has made an appointment for Septimus to see a specialist, Sir William Bradshaw, who dismisses the complexity of Septimus's madness and suggests a rest in an asylum to get more perspective.
Meanwhile, Richard Dalloway has been to lunch with Lady Bruton. Clarissa was somewhat miffed that Lady Bruton invited only Richard and not her, and sees it as a remark on Clarissa's validity. Richard has realized during this lunch that he wants to come home and tell Clarissa that he loves her. Unfortunately, he never finds the words, as he has gone so many years without saying them.
Clarissa goes to see Elizabeth, who is studying with her tutor, Doris Kilman. Clarissa despises Doris, who she sees as a monster with "hooves" taking her daughter from her. Doris also despises Clarissa, largely for her bourgeois ways and financial means.
Septimus and Lecrezia go to their apartment to wait for the attendants who will take him to the asylum. When they arrive, Septimus decides to escape from them, and not wanting to leave life but not wanting to meet the attendants, he jumps out the window to his death.
Clarissa's party is underway, with several ghosts from her past – including Peter Walsh and Sally Seton – in attendance. Richard has still been unable to tell her that he loves her. Very late into the party, Sir William and Lady Bradshaw arrive, very apologetic for their tardiness. Lady Bradshaw explains that they were delayed as one of Sir William's patients (Septimus) had committed suicide that day. The party ends with Clarissa surprisingly disappointed at the success of her party.