In the home of Wesener, a merchant who has moved to Lille with his daughters from Armentières. Charlotte, the elder sibling, is embroidering as her sister Marie writes a letter to the mother of Stolzius, her fiancé back in Armentières. Desportes, a young military officer and nobleman, visits Wesener’s shop, intent on seducing Marie. Desportes asks her father for permission to take her to the theatre, but he refuses. Alone with his daughter, Wesener explains that soldiers are famous for ruining poor girls’ reputation. Outside the barracks, the soldiers discuss the influence of the theatre on their behaviour. Haudy argues that theatre is more fun than any sermon, while Eisenhardt believes that soldiers should be subject to the same moral restrictions as civilians. At home, Wesener insinuates to Marie that a relationship with the captain could be socially beneficial to the family, and suggests that she should not tell Stolzius anything yet. Left alone, Marie can’t help but feel guilty.
In Madame Roux’s café, soldiers drink and play cards. Pirzel argues with Eisenhardt, who takes a dim view of the soldiers’ mockery of Stolzius based on rumours about his fiancé Marie. When Haudy enters the café with Stolzius, the soldiers make ironic remarks. Marie tells Desportes that Stolzius is angry with her, and Desportes offers to write a letter in reply. Marie decides to break up with Stolzius and surrender to Desportes. The situations overlap. On the one hand, Stolzius’ mother chastises him for being in love with a «soldier’s whore», while on the other, Wesener’s elderly mother sings a folksong about lost innocence. Stolzius swears to take revenge against Desportes.
Eisenhardt and Pirzel chat about the arrival of Desportes’ friend, Captain Mary. Stolzius visits Mary after deciding to join the army under Mary for the sole purpose of murdering Desportes. At the Wesener home, Charlotte chastises Marie for accepting Mary’s gallantry. She replies that Desportes has abandoned her and that this is the only way to keep in touch with him. As Charlotte argues with her sister, she is interrupted by Mary, who has come to invite her to take a walk. They all leave, accompanied by Stolzius, unrecognised by Marie. Countess De la Roche waits impatiently for her son to come home. He started a relationship with Marie shortly after she was dumped by Mary. The Countess gives him a severe dressing down, and advises him not to go on seeing a lower class girl. At Wesener’s house, the sisters argue over Marie’s attempts to make Mary jealous by accepting the young count’s gifts. They are visited by the Countess, who cruelly confronts Marie with the truth: a relationship with her son is impossible.
Marie’s past, present and future overlap in her nightmares. Stolzius decides to poison Desportes. As he serves the table where Mary and Desportes are sitting, the officers jeer about Marie. Tired of her whims and convinced that «she is no better than a whore», they left her in the hands of Desportes gamekeeper, who raped her. Stolzius reveals his identity when the poisoned soup takes effect, and then commits suicide. Endless lines of soldiers and fugitives cross paths in the city limits. Wesener meets his daughter on the street. She is now a beggar, and he does not recognize her.