The story is about a single day in the lives of three women: book editor Clarissa Vaughan in New York's West Village in 1999; novelist Virginia Woolf in Richmond, England, in 1923; and housewife and mother Laura Brown in Los Angeles in 1949.

Act 1
The chorus sings fragments of the opening line of Virginia's novel Mrs. Dalloway (the working title of which was The Hours): "Mrs. Dalloway said she would buy the flowers herself."

Clarissa and her partner Sally are preparing their apartment for a party in honor of Clarissa's best friend, Richard, who later that day is intended to receive an award for his recently published novel. Richard is seriously ill with AIDS, and Sally questions whether he will be able to come. Clarissa insists he will be able to attend. Clarissa goes out to buy flowers at a nearby shop. On her way she passes through Washington Square and notices the unusual singing of a Man Under the Arch. By chance, she encounters Walter, a writer of gay romances who knows Richard and whom she invites to the party. Walter also expresses doubt that Richard is well enough to attend the party.

In her house in the London suburb of Richmond, Virginia has awakened and entered her office, anxious to begin her new novel. Her husband Leonard, a proof editor, is concerned about her health and tries to get her to eat some breakfast, but she refuses. She considers the many roles he plays in her life.

Clarissa arrives at the flower shop, and the florist Barbara kisses her on the mouth. Clarissa briefly imagines what it would be like, if Barbara and she were lovers, and they would never need to leave the shop. She finds the flowers she wants and departs for Richard's apartment.

In her office, Virginia finds it difficult to begin her novel and thinks of central London and its many diversions, so different from her drab existence in suburban Richmond. Overcoming her hesitation, she starts to write.

In Los Angeles in her bed, Laura Brown is reading Mrs. Dalloway. She feels guilty that she is avoiding her duties as a wife and mother. Although it is her husband Dan's birthday, and he and their six-year-old son Richie are waiting for her in the kitchen, she resumes reading. Eventually, she goes down to the kitchen, where Dan and Richie express worries about her. Dan leaves for work.

Clarissa considers whether her relationship of almost eighteen years with Sally is fulfilling enough. Virginia cannot decide which of the characters in her novel will die. Laura allows Richie to help her bake Dan's birthday cake, even though Richie will surely spoil it, and feels even her efforts are inadequate.

Clarissa stops on the street on her way to Richard's and remembers the time she casually broke off their romantic relationship with simple, thoughtless words. She wonders what might have been if she had not done so.

She enters Richard's apartment to remind him of the party but finds him weak and forgetful. He calls her Mrs. Dalloway, since her first name is the same as the novel's main character. She dislikes the nickname because it is such a tragic story. He says he is unable to attend the party, but Clarissa reproaches him for not making more of an effort. He confesses he still fantasizes about whether they could have been lovers. Clarissa says she needs to leave and return to her apartment to put the flowers in water so they will not wilt.

In her kitchen, Laura is unhappy with the progress of the cake.

Virginia asks her cook Nelly whether a young girl who started the day happily could decide to commit suicide. Nelly says, if the girl became despairing, it was possible. Virginia engages in a suicidal fantasy, foreseeing her death.

Kitty unexpectedly visits Laura. The doctor has told her she has a growth inside her and needs to look at it. Can Laura feed her dog? Laura is sorry for Kitty, feels tenderness for her, and holds her. They touch their lips together, but after a moment Kitty pulls away.

Virginia, who has become anxious, stops writing and goes outdoors. Clarissa enters her apartment. Sally is thinking about how to arrange the chairs. Clarissa, worried about Richard, decides to go back to see him. Virginia, wanting to escape Richmond, considers whether to take the train to London or drown herself in the river. Laura, feeling stifled at home, takes Richie to Mrs. Latch, his sitter, and drives toward Pasadena with no plan.

Act 2
Laura enters a room at the Normandy Hotel with her tattered copy of Mrs. Dalloway and a bottle of pills. She recalls a bizarre encounter with the Hotel Clerk. While reading Mrs. Dalloway, she imagines Virginia heading to the river to kill herself. Laura considers suicide. Virginia, distracted by the voice of the Man Under the Arch, is found by Leonard, who expresses his fear of finding her dead and having to tell her sister Vanessa of his failure to save her. Virginia is disturbed by the intensity of his distress.

On her way to Richard's, Clarissa hears a choir singing lyrics seemingly directed at her. In front of Richard's building, she encounters Louis, Richard's former boyfriend, who can't decide whether to visit him. Louis recollects a summer the three of them were in Wellfleet, when he was shut out by their closeness, Richard only wanting him for his body, but Clarissa for everything else.

In her office, Virginia hears children talking and wonders if she's going crazy. In the garden, she finds her sister Vanessa and her three children: Julian, Quentin, and Angelica. Quentin has found a sick bird, which he thinks is still alive. They decide to make a bed of grass for it to die in. Virginia identifies the bird as female because it is "a bit more drab" than the males. She begins making the bed for the bird, adding roses picked from the garden. She realizes the bird is dead and begins manically making the bed, accidentally pricking Angelica's finger with a thorn. Vanessa and her children retreat, realizing the severity of Virginia's illness.

Laura in her hotel room rebukes herself for thinking of suicide when she has a young son and a baby on the way to take care of. She decides to remain alive and return to her obligations as a mother.

Clarissa lets herself into Richard's apartment and finds him on the window sill, five stories above the ground. She attempts to persuade him to come back in. He says he only wanted to write something good, not great. Something that would touch someone. He tells her he loves her, slides off the sill, and falls outside. Clarissa hesitantly peers out the window and realizes he is dead.

Clarissa appears in the street and bends over Richard's lifeless body. The chorus can be heard repeating and reassembling words and thoughts from the day. Laura leaves the hotel. Virginia realizes she is losing her mind.

Laura picks Richie up from the sitter, and he tells her he was scared because he thought, like Kitty, she had something growing inside her.

At their dining table, Virginia expresses her gratitude to Leonard for having given her "the greatest possible happiness."

Dan returns home to his birthday party and tells his family how happy they have made him.

At Sally and Clarissa's, the party has become a wake. Richard's mother, Laura, arrives and reveals she abandoned Dan and Richie and feels regret.

As the others leave, the three female protagonists, Laura, Clarissa, and Virginia sing a trio, realizing that in their connection to one another across different times and places, they are not alone.