A day in the life of two individuals moving separately through London, one in a state of hallucinatory detachment, one buoyant, social, and comfortable with the pleasures of wealth: 13 June, 1923. While Septimus Warren Smith, a shell-shocked veteran of the War, hallucinates in the park, Clarissa Dalloway, the perfect hostess, is out on errands, buying flowers in advance of her party. She loves being out and about:

In people's eyes, in the swing, tramp, and trudge; in the bellow and the uproar; the carriages, motor cars, omnibuses, vans, sandwich men shuffling and swinging; brass bands; barrel organs; in the triumph and the jingle and the strange high singing of some aeroplane overhead was what she loved; life; London; this moment of June.

There's an explosion as a motor car backfires, and the people outside the flower shop turn to look.

Passers-by who, of course, stopped and stared, had just enough time to see a face of the very greatest importance against the dove-grey upholstery, before a male hand drew the blind [...] But nobody knew whose face had been seen. Was it the Prince of Wales's, the Queen's, the Prime Minister's?

Clarissa and Septimus are momentarily linked as they watch the motor car roll down Bond Street towards Piccadilly; otherwise their paths through the city are independent. Septimus's doting wife Lucrezia tugs him away from the spectacle as he begins to mutter suicidally and leads him to Regent's Park to relax.

Following oddly on the mysterious car's progress, an airplane appears, "the sound boring into the ears of all people in the Mall, in the Green Park, in Piccadilly, in Regent Street, in Regent's Park," and they're shocked to see its puffs "making letters in the sky." This is the first time anyone has seen skywriting, and the on-lookers attempt to spell out its message (Glaxo? Kreemo? Toffee?) a letter at a time. Septimus becomes enraptured and teary watching "one shape after another of unimaginable beauty"—for once the real world is speaking his language. Auditory hallucinations unfold:

"K... R..." said the nursemaid, and Septimus heard her say "Kay Arr" close to his ear, deeply, softly, like a mellow organ, but with a roughness in her voice like a grasshopper's, which rasped his spine deliciously and sent running up into his brain waves of sound which, concussing, broke.

That evening, a friend brings news to Clarissa's party: "A young man [...] killed himself. He had been in the army. [...] He had thrown himself from a window." This fragment of information is enough to set Clarissa's imagination in motion: fascination, empathy, even envy:

Death was defiance. Death was an attempt to communicate, people feeling the impossibility of reaching the centre which, mystically, evaded them; closeness drew apart; rapture faded; one was alone. There was an embrace in death.