The main plot is the escalator ride of Howie, an office worker in his mid-twenties, at the end of his lunch break. While going up to the mezzanine floor, where his office is, he recounts his day and the thoughts he had so far. It is this incredibly detailed account of seemingly random thoughts and ideas that make up most of the book, with links to other thoughts presented in elaborate footnotes.
Howie's mission during his lunch break is to get some new shoelaces after both of his old ones snapped almost at the same time. He marvels at his seemingly consistent technique of tying his shoelaces before getting lost in thoughts about plastic straws, milk cartons, as well as elements of office culture and common courtesy. Howie exposes office smalltalk as clichéd communication without any meaning, and during his trip to the men's room carefully observes the transmission of whistled melodies while accurately describing the unwritten rules of using the facilities.
When Howie starts thinking about his thoughts, he realizes that most of his current thoughts are still heavily influenced by his childhood, so he hypothesizes that he will be a true adult only if the majority of his thoughts are based on his experiences as an adult--or if, in his words, the units of adult thoughts outweigh the units of child thoughts. Therefore, he starts to devise a taxonomy of thoughts; he quantifies them and concludes that from a cognitive standpoint he will have reached adulthood somewhere in his forties.
At the end of his lunch break, he briefly sits down on a bench eating some junk food and reading Aurelius' Meditations before he goes back into his office building and rides up the escalator.