After Éponine's release from prison, she finds Marius at "The Field of the Lark" and sadly tells him that she found Cosette's address. She leads him to Valjean's and Cosette's house on Rue Plumet, and Marius watches the house for a few days. He and Cosette then finally meet and declare their love for one another. Thénardier, Patron-Minette and Brujon manage to escape from prison with the aid of Gavroche. One night, during one of Marius's visits with Cosette, the six men attempt to raid Valjean's and Cosette's house. However, Éponine, who has been sitting by the gates of the house, threatens to scream and awaken the whole neighbourhood if the thieves do not leave. Hearing this, they reluctantly retire. Meanwhile, Cosette informs Marius that she and Valjean will be leaving for England in a week's time, which greatly troubles the pair.

The next day, Valjean is sitting in the Champ de Mars. He is feeling troubled about seeing Thénardier in the neighbourhood several times. Unexpectedly, a note lands in his lap, which says "Move Out." He sees a figure running away in the dim light. He goes back to his house, tells Cosette they will be staying at their other house on Rue de l'Homme Arme, and reconfirms to her that they will be moving to England. Marius tries to get permission from M. Gillenormand to marry Cosette. His grandfather seems stern and angry, but has been longing for Marius's return. When tempers flare, he refuses his assent to the marriage, telling Marius to make Cosette his mistress instead. Insulted, Marius leaves.

The following day, the students revolt and erect barricades in the narrow streets of Paris. Gavroche spots Javert and informs Enjolras that Javert is a spy. When Enjolras confronts him about this, he admits his identity and his orders to spy on the students. Enjolras and the other students tie him up to a pole in the Corinth restaurant. Later that evening, Marius goes back to Valjean's and Cosette's house on Rue Plumet, but finds the house no longer occupied. He then hears a voice telling him that his friends are waiting for him at the barricade. Distraught to find Cosette gone, he heeds the voice and goes.

When Marius arrives at the barricade, the "revolution" has already started. When he stoops down to pick up a powder keg, a soldier comes up to shoot Marius. However a man covers the muzzle of the soldier's gun with his hand. The soldier fires, fatally shooting the man, while missing Marius. Meanwhile, the soldiers are closing in. Marius climbs to the top of the barricade, holding a torch in one hand, a powder keg in the other, and threatens to the soldiers that he will blow up the barricade. After confirming this, the soldiers retreat from the barricade.

Marius decides to go to the smaller barricade, which he finds empty. As he turns back, the man who took the fatal shot for Marius earlier calls Marius by his name. Marius discovers this man is Éponine, dressed in men's clothes. As she lies dying on his knees, she confesses that she was the one who told him to go to the barricade, hoping they would die together. She also confesses to saving his life because she wanted to die before he did.

The author also states to the reader that Éponine anonymously threw the note to Valjean. Éponine then tells Marius that she has a letter for him. She also confesses to have obtained the letter the day before, originally not planning to give it to him, but decides to do so in fear he would be angry at her about it in the afterlife. After Marius takes the letter, Éponine then asks him to kiss her on the forehead when she is dead, which he promises to do. With her last breath, she confesses that she was "a little bit in love" with him, and dies.

Marius fulfills her request and goes into a tavern to read the letter. It is written by Cosette. He learns Cosette's whereabouts and he writes a farewell letter to her. He sends Gavroche to deliver it to her, but Gavroche leaves it with Valjean. Valjean, learning that Cosette's lover is fighting, is at first relieved, but an hour later, he puts on a National Guard uniform, arms himself with a gun and ammunition, and leaves his home.