Antoine Roquentin, a historian living in Bouville, France, begins a diary to help him explain the strange and sickening sensations that have been bothering him for the previous few days. He is not sure what exactly is wrong with him, often doubting if there is any need to keep a diary at all. Nevertheless, a few days later, he is so overcome with what he calls the Nausea, that he begins to furiously list every insignificant fact, detail, feeling, and impression occurring both inside himself and the outside world. He holds a stone, looks at a glass of beer, and tries to touch a soggy piece of paper in the street--each time sensing a worrisome and overwhelming presence.
For the past ten years, Roquentin has been researching the Marquis de Rollebon, a French aristocrat who lived during the French Revolution. Rollebon was originally from Bouville so Roquentin moved there to complete his research and write a book about him. But his feelings of Nausea soon extend to his research. Whenever he looks in a mirror, he is not sure if he sees his own face or Rollebon's. He soon loses interest in his work, realizing that he can never understand Rollebon as if he were still alive. Roquentin feels constrained by the past, choosing instead to live in the present.
Roquentin begins to understand that his feelings of Nausea have something to do with the question of existence. He realizes that he had been using Rollebon and the past in general to justify his own existence. Roquentin defiantly asserts his own existence, claiming that everyone else he sees is afraid to acknowledge that they exist. Focusing on the existence of objects and people, Roquentin discovers that "existence precedes essence." Looking at the root of a chestnut tree, he realizes that his perception of the root's essence, or its physical characteristics, in fact hides the truth of the object's existence. The comforting facade of tastes, colors, smells, weight, and appearance are thus the creation of the observer. Looking through the essence of objects, Roquentin is confronted with the bare existence of things, and thus the source of his Nausea.
Roquentin visits his ex-lover Anny in Paris. He had hoped that they would get back together, but is disappointed to find that they do not communicate very well. He vainly attempts to explain his feelings of Nausea to Anny, but she does not understand. They part, knowing that they will never see each other again. Back in Bouville, Roquentin resolves to free himself from the past by embracing his existence in the present. He tries to explain his views to the Self-Taught Man, a lonely cafe acquaintance, but he cannot persuade him that human love is just an essence, and that there is no purpose to existence, only "nothingness." Despite his despair and abandoning his research, Roquentin chooses to move to Paris and write a novel.