Les Misérables is both a book and a musical, both of which have also been rendered into films. The majority of badfic is based on the musical, as fanbrats can't be bothered to read the Brick (an affectionate fan-nickname for the 1200-page tome).
The novel was written by Victor Hugo in 1862, and has been translated into English six times. The main plot (although there are countless sub-plots) concerns a convict named Jean Valjean who is released from prison and then attempts to become an honest man through performing good works. However, Valjean cannot escape the shadow of his past, as personified by Inspector Javert, who pursues him relentlessly throughout the course of the novel.
The conflict between the two men provides the backbone of the book, but woven around it are many other plots: the love between Marius (a young student) and Cosette (Valjean's adopted daughter) and Eponine's unrequited pursuit of the former; the student revolutionaries (Les Amis de l'ABC) and the revolution itself; and the story of the Thénardier family (Eponine, Gavroche, Azelma, Monsieur Thénardier, and Madame Thénardier).
The Brick also contains numerous chapter-long digressions, including a re-telling of the Battle of Waterloo and a lecture on religion. These are usually skipped on the second read-through by all but the most dedicated of fans. It contains at least one sentence that extends over a page and a half, linked by an excessive number of semicolons.
The musical's plot does not differ much from that of the book, except for the removal of many of the extraneous plotlines and the condensation of the action (and no digressions about the Battle of Waterloo).
Mary Sues in this fandom tend to gravitate either towards Marius or Enjolras, since they are the young and handsome characters. However, nearly all of Les Amis de l'ABC have been pursued by Sues of one sort or another, and there are also a surprising number of Javert-loving Sues out there (all of whom seem to forget about the age difference). Bad slash between various male characters is also not unheard of.