Jean Valjean has been on edge ever since the run-in with Thénardier. He decides that it's time to get out of Paris and go to England. And one day, while siting on a bench, he sees a piece of folded paper fall onto his knee. Glancing up, he sees a young girl running away. The piece of paper simply says, "CLEAR OUT." That pretty much clinches it. Time to jet.
Meanwhile, we pick up with Marius leaving his grandfather's house. That night, he returns to Cosette's garden to meet her. But she never shows up. While he's busy feeling sorry for himself, someone whispers through a hedge that his friends from the ABC Society are waiting for him in a downtown street. It sounds like the radicals have set up a public barricade.
In another part of Paris, the old Monsieur Mabeuf hears about the barricade and the rioting in downtown Paris and decides that he's going to go check it out. He has no money left and has sold all his books to feed himself, so he figures there's nothing left for him to do but go where the action is.