For a shorter summary with more pictures, take a look at the characters in the game.
The plot of Les Miserables (warning! spoilers ahead!)
Les Misérables is set in France in the first half of the nineteenth century, and tells the story of the miserable poor (hence the title). At over 1200 pages, the book contains a large number of different plots, all linked by the story of Jean Valjean. Valjean was jailed for nineteen years for stealing food for his starving family (with extra years for trying to escape) and this destroyed his soul. He entered as a man who cared for his family, but when he left he was bitter and dead inside.
Valjean was given a new reason to live by the kindness of a bishop, and dedicated his life to doing good. However, he had technically broken the terms of his parole, and a policeman, Javert, spent the rest of his life trying to find him and bring him to justice.
Living under an assumed name in a different town, Valjean became a wealthy factory owner and was appointed mayor, but his past caught up with him and he was forced to run again. In a final climax he saves the life of Javert who is forced to reevaluate his own views, with memorable consequences. Meanwhile, Valjean failed to save the life of a tragic woman Fantine, and adopted her daughter Cosette. Cosette grows up and falls in love with a revolutionary, Marius, who is in turn loved by the daughter of the only real villain in the novel, Thenardier. During a violent seige at the barricades Marius is wounded and dying, when... well you get the idea. A lot happens in the book. Much of that is covered in the famous musical, and even more of it is included in the game.
Victor Hugo made it clear that the message of the novel was social justice and the need to reform the terrible state of the poor. Rather than try to improve on Victor Hugo, the game is dedicated to the same message, told through the dramatic events in the lives of Valjean, Fantine, Marius, and the others.
Most people consider that Les Miserables is one of the greatest novels of all time. The only real criticism of the novel is that it relies on coincidences. People happen to be in the same place at the same time, Valjean stumbles on a way to be rich, at one point he just bumps into the only person who can save someone's life at the last possible moment, and so on. This is where the game comes in. In the game your job is to make these things happen. Valjean, and the people who rely on him, rely on you.
A Les Miserables timeline
Combining real events and events in the book.
1753: Jean Valjean is born. This is only mentioned in flashback.
1778: Jean Valjean's father dies, and Jean becomes responsible for his family.
1789: The storming of the Bastille - the most famous event in the French Revolution. The First Republic.
1793: The "great terror", where the revolutionaries kill thousands.
1795: Valjean steals a loaf of bread to feed his sister's starving children. He is imprisoned.
1802: (Victor Hugo is born)
1804: M. Myriel is curate of Brigolles. Napoleon is declared Emperor and begins to conquer Europe.
1806: M. Myriel becomes bishop at Digne.
1812: Napoleon has his greatest defeat, in Russia.
1814: Napoleon has further setbacks, and is finally defeated in 1815. The monarchy is restored, but with less power than before.
1815. The first chapters of Les Miserables: Valjean is released from prison.
1816? Cosette is born.
1820: Valjean is at the height of his success. M. Myriel dies.
1823: Valjean is identified and imprisoned again. He later escapes.
1824: After escaping and living in Paris for a while, he becomes a gardener at a convent.
1828: Marius learns about his father and becomes friends with revolutionaries.
1829: Valjean leaves the convent and buys a house at the Rue Plume.
1830: The "July Revolution" fails, giving more power to the king.
1832: An economic downturn increases hardships among the ordinary people. The government does not handle the situation well. The death of a much loved general, Lamarque, is the spark that ignites the building of the barricades. The 6th of June 1832 is the climax of Les Miserables.
1833: The last chapters of Les Miserables. Cosette is married. Later that year Valjean is vindicated, and dies, aged 80. Until that year his great health made him look just fifty, but the events of his last year aged him.
1848: The last major revolution in France: the monarch is overthrown and the Second Republic begins.
1852: Emperor Napoleon III gains power and ends the Second Republic.
1875: The Third Republic is declared.
1885: Victor Hugo dies.
Plot spoilers:
what happens in the book and the game