The story revolves mainly around the characters of Renée Michel and Paloma Josse, residents of an upper-middle class Left Bank apartment building at 7 Rue de Grenelle – one of the most elegant streets in Paris. Divided into eight luxury apartments, all occupied by distinctly bourgeois families, the building has a courtyard and private garden.
The widow Renée is a concierge who has supervised the building for 27 years. She is an autodidact in literature and philosophy, but conceals it to keep her job and, she believes, to avoid the condemnation of the building's tenants. Likewise, she wants to be alone to avoid her tenants' curiosity. She effects this by pretending to indulge in concierge-type food and low-quality television, while in her back room she actually enjoys high-quality food, listens to opera, and reads works by Leo Tolstoy and Edmund Husserl. Her perspective is that "[t]o be poor, ugly and, moreover, intelligent condemns one, in our society, to a dark and disillusioned life, a condition one ought to accept at an early age".
Twelve-year-old Paloma lives on the fifth floor with her parents and sister whom she considers snobs. A precocious girl, she hides her intelligence to avoid exclusion at school. Dismayed by the privileged people around her, she decides that life is meaningless, and that unless she can find something worth living for, beyond the "vacuousness of bourgeois existence", she will commit suicide on June 16, her thirteenth birthday. Planning to burn down the apartment before dying, she also steals her mother's pills. For the time being she journals her observations of the outside world, including her perceptions of Renée.
Paloma is the only tenant who suspects Renée's refinement, and for most of the novel, the two "cross each other but don’t see each other", in the words of Time Out reviewer Elisabeth Vincentelli. Although they share interests in philosophy and literature, nothing happens between them until the death of a celebrated restaurant critic who had been living upstairs. A cultured Japanese businessman named Kakuro Ozu, whom Renée and Paloma befriend, then takes a room in the same apartment building. Ozu comes to share Paloma's fascination with Renée: that the concierge has the "same simple refinement as the hedgehog".
Towards the end of the novel, Renée comes out of her internal seclusion, teaching young Paloma that not all adults pursue vanity at the expense of their intelligence and humanity. However, only shortly after Renée realizes that the beauty of life and her connections with the world makes life worth living, she dies in the same way as Roland Barthes; she is struck down by a laundry van. This leaves Paloma and Ozu devastated but leads Paloma not to commit suicide.