Claude Frollo corners Pierre Gringoire after one of Gringoire's ridiculous street performances. He tells Gringoire that the Parliament has decreed that La Esmerelda is to be forcibly removed from Notre Dame and hanged within three days. Gringoire doesn't seem to care much, exclaiming that he will miss her goat, Djali. Frollo reminds Gringoire that La Esmerelda saved his life and that they were technically married before she was arrested. Gringoire is still reluctant to risk his life to save her and declares that the last thing he wants is to be hanged. Frollo then tries to convince him with philosophical arguments and gets Gringoire to agree that one should die as one has lived. Gringoire tricks himself into thinking that he was predestined to die saving La Esmerelda and he runs off to enlist the help of the other vagabonds. Frollo then runs into his brother, Jehan, who claims to have repented for being such a horrible student and regrets not having heeded his brother's advice and warnings earlier in life. He then asks Claude for money and, when Claude refuses, Jehan runs off, declaring that he has decided to join the vagabonds.
Jehan has little trouble convincing the vagabonds to let him join their ranks. They are busy preparing an assault on Notre Dame to rescue La Esmerelda and when Jehan tells him that he knows his way around the cathedral, they eagerly accept him. The vagabonds, led by their "King," Clopin Trouillefou, declare that La Esmerelda is their sister and that they will not let her die. Gringoire informs Clopin that Louis XI is supposedly in Paris, which makes the vagabonds even more excited about the prospect of saving La Esmerelda before the King of France's very eyes. The midnight bells chime and a large group of vagabonds, made up of men, women and children, set out for Notre Dame in the dark. Meanwhile, Quasimodo is surveying the streets of Paris from the top of the cathedral. Looking toward the Seine, he spies a strange black shape moving along the riverbank. He gradually makes out a procession of armed vagabonds heading directly for Notre Dame. Recalling rumors that La Esmerelda's life is in danger, Quasimodo thinks that the vagabonds are going to try to kill her and makes plans to defend the cathedral from the imminent assault.
Before leading the vagabonds into battle, Clopin cries out that they have come to rescue La Esmerelda and that they will not take no for an answer. Unfortunately, Quasimodo can't hear them and he assumes that they have come to kill La Esmerelda. The vagabonds start to saw and hammer away at the cathedral's front door, desperately trying to hammer it open. All by himself, Quasimodo decides to defend Notre Dame single handedly. He grabs a large wooded beam from the belfry and throws it down from the tower, crushing and killing a large number of vagabonds below. This sudden counter-attack frightens half of Clopin's troops away and Quasimodo thinks that he has successfully defended the cathedral. But the remaining vagabonds, intent on pillaging the cathedral, pick up the beam and start to use it as a battering ram against the front door. Despite the iron casing around the door, it gradually starts to break. Quasimodo then finds some rolls of lead, melts them, and then pours them into rain gutters that consequently spew out hot molten lead on the vagabonds. The majority of them burn alive under the shower of lead. Jehan is one of the few vagabonds left and he tries to climb up a ladder leading to a side entrance. Quasimodo pushes back the ladder and grabs Jehan by the legs. He swings Jehan up against a wall, crushing his skull and then throws him down to his death. The vagabonds are outraged. They start to climb up the cathedral's facade and Quasimodo fears that he will soon be overwhelmed.
Meanwhile, Louis XI, the King of France, has been monitoring the situation. He is an old and bitter monarch who has a bad reputation among the people. Gringoire is brought to him as a prisoner and he pleads for mercy, reminding the King that clemency is a noble virtue. Louis agrees to let him go, but only on the condition that he help them hanger the "sorceress." Back at Notre Dame, Quasimodo is about to surrender when Phoebus de Chateaupers and the King's Archers arrive and save the day. They clear out the remaining vagabonds and storm the cathedral to arrest La Esmerelda. Quasimodo thinks they have come to save her and runs to her cell to bring her the good news. To his horror, she is nowhere to be found.
The vagabond's assault is a thrilling battle scene, a comic interlude, and a class conflict all at the same time. The vagabonds fancy themselves an army when they have absolutely no clue what they are doing. They all profess a sincere devotion to La Esmerelda but are really just motivated by the idea of stealing all of Notre Dame's silver and gold. Nevertheless, as they prepare to march, Clopin declares: "Trade is incompatible with nobility." This subtle but powerful statement of purpose adds an extra historical dimension to their attack. Hugo was profoundly marked by the class differences that set the 1789 French Revolution in motion. Discord between the Clergy, Nobility, and the Third Estate (or middle class of artisans, craftsmen, and intellectuals) toppled the monarchy and established a republican government that no longer recognized the special privileges of the aristocracy and the Church. Writing during the July 1830 Revolution, Hugo was more conscious of class divisions than ever. Consequently, the vagabonds assault on Notre Dame represents an example of historical foreshadowing that would remind Hugo's contemporary readers of the 1789 storming of the Bastille. The fact that Louis XI is in the Bastille at the same time as the vagabonds' attack further emphasizes this historical reference.
Hugo digresses and give the reader more historical background on the Middle Ages. He explains the vagabonds' decision to attack Notre Dame by pointing out that such violent assaults were common during the Middle Ages because there was no police force. Paris was operated under the feudal system, meaning that there was no central regulating power. Paris was an assemblage of a thousand seigneuries that divided and subdivided the city into a thousand different "compartments," resulting in a thousand contradictory security forces, each with different agendas and allegiances. For example, just in the Church lands alone, bishops and priests would regulate a set number of streets that were in different jurisdictions than land controlled by the King and his vassals. Louis XI tried to do away with the feudal system and impose a curfew but the outcome was so confusing that no one was sure which "watch" or "counter-watch" was legitimate. This context thus adds historical legitimacy to Hugo's description of the vagabonds' attack.
Quasimodo demonstrates his physical and spiritual affinity with Notre Dame during the attack. He is the cathedral's only defender, but he knows exactly what to do. The architecture of Notre Dame becomes an extension of the hunchback's own body, as the beam and gargoyle spouts all represent parts of the cathedral that Quasimodo knows how to use to the utmost precision. He defends Notre Dame well because it is his home, and he is tragically unaware that he is killing the only people with which he could trust La Esmerelda's life. This scene's horrific violence climaxes with the gruesome death of Jehan. The narrator compares the sound of Jehan's head slamming aginst the wall to a "crushed coconut." His tragic and gory death foreshadows Claude Frollo's demise. Jehan had been Frollo's only reason to live and it is doubly tragic that his second "adopted son," Quasimodo," kills his step-brother. Quasimodo's love for La Esmerelda drives him to such violent ends, mirroring Frollo's own violent acts to win her love.