One day, a guy named Antoine Roquentin decides to start writing a diary because he's worried that he's going crazy. For that reason, he wants to record all of his thought in order to understand how he might be changing—changing, perhaps, into a total nutbag.
He claims that a sort of mental sickness or "nausea" is starting to take over his brain and body, although he doesn't know where it has come from—it's not like he's eaten bad shellfish lately. He spends most of his time trying to distract himself from this feeling by having casual sex and working on a personal history project. But no matter how hard he tries his feelings of dread keep coming back. Whoops.
While leaving his apartment one day, Antoine gets a note from his ex lover Anny saying that she'll be in town soon and she wants to see him. Antoine feels like looking forward to this meeting is the only thing that gets him through the days, especially since he's becoming more and more bored with his history project. When Anny finally does show up, though, she tells him that she's with a new man and it's too late for him to get her back. Whoops again.
Toward the end of the book, Antoine realizes that his "nausea" isn't a mental illness, but just a side effect of him figuring out the truth of human existence. And what is this truth, you ask? Nothing… other than the fact that the universe is completely indifferent to human life. Humans are completely free to do whatever they want, Antoine thinks, because there's no God and human life does not have any higher purpose. There are only individuals who find meaning in their own unique ways.
On the one hand, this truth is good because it means individuals are totally free. On the other hand, it's sad because life doesn't have any intrinsic meaning, only the meaning that we choose to give it.