Enjolras and a buddy are holding their rifles when they hear Gavroche yelling that the French army is on its way and that some serious fighting is about to go down.
The ABC folks and their partners (43 people in total) take their positions behind the barricade. The army offers them a chance to surrender, but Enjolras loudly rejects it. Next thing you know, there bullets flying into the barricade. Luckily, none of them can make it through. However, the group's revolutionary flag has fallen in the process, and someone needs to go get it. But surprise surprise, no one wants to risk getting shot to save a flag.
The person who eventually emerges from the crowd is Monsieur Mabeuf. He climbs down to get the flag and inspires everyone behind the barricade. Even the French soldiers stop shooting for a moment. But then the old man starts waving the flag and shouting revolutionary slogans, and the army guns him down.
Enjolras removers the old man's bloodstained coat and declares that it will be the new flag of the revolution.
After Mabeuf's death, the French army charges the barricade. Gavroche and Courfeyrac are in serious dangers until Marius shows up at the last second and blows away the men attacking them. But now his weapons are empty.
A French soldier fires at Marius, but at the last second, a young person jumps in front of the shot and takes it for him. Marius isn't even aware of what just happened, and he runs into the tavern. When he comes back out, he seizes a giant powder keg (or in other words, a bomb) and tells the attacking army that he'll blow all of them (including himself) to kingdom come if they don't retreat. The men don't want to call his bluff, so they retreat.
With the attacking soldiers gone, the ABC folks crowd around Marius and pat him on the back, thanking him for saving their lives.
Unfortunately, the attacks took a prisoner while they were on the barricade, a young poet named Jean Prouvaire. After hearing them execute Prouvaire, Enjolras turns to the captive Javert and says that his army friends have just gotten him killed.
Meanwhile, Marius finds the young man who took his bullet dying outside. On further inspection, Marius realizes that the young man is actually a young woman—none other than Éponine.
In case we haven't figured it out yet, Éponine has always had a crush on Marius. Before she dies, she apologizes to Marius and pulls out a letter from Cosette that she was supposed to give to him. She held onto it because she wanted Cosette out of Marius' life.
As her dying wish, Éponine asks Marius to kiss her forehead after she's dead. And he does, the big softy.
With that done, Marius reads the letter Éponine held back from him. It's from Cosette and it says she'll be staying in a certain part of Paris for the next week before she leaves for England. Marius decides that he must find Cosette. First though, he writes his own response, gives it to Gavroche, and asks him to sneak out of the barricade and bring it to Cosette's address. Gavroche doesn't want to leave the action, but he does.