In Les Miserables, Marius and the other revolutionaries are part of a group called the friends of the ABC which is led by a character called Enjolras, who want to give people liberty, equality and fraternity (the saying of the French Revolution). The story takes place during the battles following the French revolution, so Marius is following his fathers footsteps since he fought in Napoleon's army. Basically, they want to help the people of France get better social, economic and political status like the above comment says.
I haven't gotten through the whole book of Les Miserables yet, but it's my favorite musical so I will summarize that. It's similar to the book plot but has many differences too. The main character, Jean Valjean, stole a loaf of bread when he was younger to help his family. The story begins when his parole has begun. When he leaves he can't find anywhere to stay until a bishop kindly feeds him and lets him spend the night, but Valjean steals silverware and runs away in the middle of the night. When the police catch him and bring him back, the bishop says he gave the silver to him willingly to save Valjean, who promises he will now become an honest, good man... So, Jean breaks his parole but uses the silver to become a rich mayor. Fantine is a worker that lives in the town he works for and is soon told to leave by other workers and the harsh inspector Javert after they learn she has a child living with another family (the Thenardiers) that she can't afford/ take care of her because the father abandoned them. She becomes a prostitute when she is fired but is arrested because a man says she attacked him. Jean Valjean promises to take Fantine's daughter, Cosette, but now he is on the run because Javert realizes he is Valjean, who should still be in jail. He gets away and takes Cosette, hiding constantly from Javert. Cosette grows up with Valjean because Fantine dies right before he gets her, and she meets Marius. They quickly fall in love, but he has to fight for France. Meanwhile, Marius' best friend Eponine who is secretly in love with him becomes very depressed and jealous. Jean tells Cosette they must leave again and before fighting, Marius learns this and sends Eponine to give her a letter. When Eponine returns to Marius she is shot and dies in his arms. This motivates the students to fight, but in the end Marius is basically the only one left. He is wounded, but Valjean saves him and decides he and Cosette should get married (at the beginning he dislikes Marius). Cosette learns about her past. Jean Valjean dies, but it is after he becomes a good and honest man :) That's about as brief as I can make it...hope it helps!
I LOVE LES MIS :)