Eight years later, the Friends of the ABC, led by Enjolras, are preparing an act of anti-Orléanist civil unrest on the eve of the Paris uprising on 5–6 June 1832, following the death of General Lamarque, the only French leader who had sympathy towards the working class. They are also joined by the poor of the Cour des miracles, including the Thénardiers' eldest son Gavroche, who is a street urchin.

One of the students, Marius Pontmercy, has become alienated from his family (especially his grandfather M. Gillenormand) because of his liberal views. After the death of his father Colonel Georges Pontmercy, Marius discovers a note from him instructing his son to provide help to a sergeant named Thénardier who saved Pontmercy's life at Waterloo – in reality Thénardier was looting corpses and only saved Pontmercy's life by accident; he had called himself a sergeant under Napoleon to avoid exposing himself as a robber.

At the Luxembourg Gardens, Marius falls in love with the now grown and beautiful Cosette. The Thénardiers have also moved to Paris and now live in poverty after losing their inn. They live under the surname "Jondrette" at Gorbeau House (coincidentally, the same building Valjean and Cosette briefly lived in after leaving the Thénardiers' inn). Marius lives there as well, next door to the Thénardiers.

Éponine, now ragged and emaciated, visits Marius at his apartment to beg for money. To impress him, she tries to prove her literacy by reading aloud from a book and by writing "The Cops Are Here" on a sheet of paper. Marius pities her and gives her some money. After Éponine leaves, Marius observes the "Jondrettes" in their apartment through a crack in the wall. Éponine comes in and announces that a philanthropist and his daughter are arriving to visit them. In order to look poorer, Thénardier puts out the fire and breaks a chair. He also orders Azelma to punch out a window pane, which she does, resulting in cutting her hand (as Thénardier had hoped).

The philanthropist and his daughter enter—actually Valjean and Cosette. Marius immediately recognizes Cosette. After seeing them, Valjean promises them he will return with rent money for them. After he and Cosette leave, Marius asks Éponine to retrieve her address for him. Éponine, who is in love with Marius herself, reluctantly agrees to do so. The Thénardiers have also recognized Valjean and Cosette, and vow their revenge. Thénardier enlists the aid of the Patron-Minette, a well-known and feared gang of murderers and robbers.

Marius overhears Thénardier's plan and goes to Javert to report the crime. Javert gives Marius two pistols and instructs him to fire one into the air if things get dangerous. Marius returns home and waits for Javert and the police to arrive. Thénardier sends Éponine and Azelma outside to look out for the police. When Valjean returns with rent money, Thénardier, with Patron-Minette, ambushes him and he reveals his real identity to Valjean. Marius recognizes Thénardier as the man who "saved" his father's life at Waterloo and is caught in a dilemma.

He tries to find a way to save Valjean while not betraying Thénardier. Valjean denies knowing Thénardier and tells him that they have never met. Valjean tries to escape through a window but is subdued and tied up. Thénardier orders Valjean to pay him 200,000 francs. He also orders Valjean to write a letter to Cosette to return to the apartment, and they would keep her with them until he delivers the money. After Valjean writes the letter and informs Thénardier of his address, Thénardier sends out Mme. Thénardier to get Cosette. Mme. Thénardier comes back alone, and announces the address is a fake.

It is during this time that Valjean manages to free himself. Thénardier decides to kill Valjean. While he and Patron-Minette are about to do so, Marius remembers the scrap of paper that Éponine wrote on earlier. He throws it into the Thénardiers' apartment through the wall crack. Thénardier reads it and thinks Éponine threw it inside. He, Mme. Thénardier and Patron-Minette try to escape, only to be stopped by Javert.

He arrests all the Thénardiers and Patron-Minette (except Claquesous, who escapes during his transportation to prison; Montparnasse, who stops to run off with Éponine instead of joining in on the robbery; and Gavroche, who was not present and rarely participates in his family's crimes, a notable exception being his part in breaking his father out of prison). Valjean manages to escape the scene before Javert sees him.