The story follows a group of French residents of Rouen, recently occupied by the Prussian army. The ten travellers decide for various reasons to leave Rouen and flee to Le Havre in a stagecoach. Sharing the carriage are Boule de Suif or "Butterball" (lit. suet dumpling, also translated as ball of fat), a prostitute whose real name is Elisabeth Rousset; the strict Democrat Cornudet; a shop-owning couple from the petty bourgeoisie, M. and Mme. Loiseau; a wealthy upper-bourgeoisie factory-owner and his wife, M. and Mme. Carré-Lamadon; the Comte and Comtesse of Bréville; and two nuns. Thus, the carriage constitutes a microcosm of French society, representing different parts of the French population during the late 19th century.
Due to the terrible weather, the coach moves very slowly and by midday has only covered a few miles. The occupants initially snub Boule de Suif, but their attitudes change when she produces a picnic basket full of lovely food and offers to share its contents with the hungry travellers.
At the village of Tôtes, the carriage stops at the local coaching inn, and the occupants, hearing a German voice, realise they have blundered into Prussian-held territory. A Prussian officer detains the party at the inn indefinitely without telling them why. Over the next two days, the travellers become increasingly impatient, and are finally told by Boule de Suif that they are being detained until she agrees to sleep with the officer. She is repeatedly called before the officer, and always returns in a heightened state of agitation. Initially, the travellers support her and are furious at the officer's arrogance, but their indignation soon disappears as they grow angry at Boule de Suif for not sleeping with the officer so that they can leave. Over the course of the next two days, the travelers use various examples of logic and morality to convince her it is the right thing to do; she finally gives in and sleeps with the officer, who allows them to leave the next morning.
As they continue on their way to Le Havre, these "representatives of Virtue" ignore Boule de Suif and turn to polite topics of conversation, glancing scathingly at the young woman while refusing to even acknowledge her, and refusing to share their food with her the way that she did with them earlier. As the coach travels on into the night, Cornudet starts whistling the Marseillaise which sours the mood of everyone in the coach, and all the while Boule de Suif, bemoaning her lost dignity, can hardly suppress her sobs.
