Roquentin introduces himself and begins recording an unspecified change in his relationship to things. He drops a pebble on the beach and cannot say why. The diary form is established as an attempt to fix and understand the malaise.

The malaise spreads from specific objects to general perception. A door handle, a glass of beer, a man's suspenders trigger waves of the sensation. Roquentin begins to suspect that the problem is not with the objects but with the nature of existence itself.

Roquentin has lunch with the Self-Taught Man, who reveals himself as a devout Humanist who loves all human beings as a matter of principle. Roquentin's contempt for this position is the novel's central philosophical debate — between bad-faith optimism and authentic confrontation with contingency.

Roquentin realizes he can no longer write the biography of the Marquis de Rollebon. The historical project has been a defense against his own existence, and without it he has nothing — no project, no identity, no reason to be in Bouville.

In the Public Garden, Roquentin stares at the root of a chestnut tree and has the definitive experience of existence as pure, unjustified, excessive fact. The word 'root' fails. All words fail. He understands: existence precedes essence, and existence has no essence.

Roquentin's former lover Anny arrives in Bouville. She was once obsessed with constructing 'perfect moments' — staged scenes of maximum significance. She has abandoned this project and now moves through the world performing emotions she no longer feels. They part without hope.

The Self-Taught Man is caught in the library making advances to a young boy and is publicly humiliated and ejected. Roquentin, who witnesses the scene, feels unexpected pity — and recognizes that the Self-Taught Man's humanism was itself a defense against his own desires and solitude.

Roquentin prepares to leave Bouville for Paris. In the cafe where he has spent so many afternoons, he plays his favorite jazz record one last time and has an insight: the song exists differently from a pebble or a root. It was made. Its necessity is conferred by its creator. Perhaps he could write something — not history, but a novel — that would exist in this way and would justify, retroactively, the fact of his having lived.