When we meet Fantine, she's an innocent girl who likes to hang out in Paris with her friends, probably drawing hearts all over 1D posters or whatever the nineteenth-century French equivalent was. And she's completely in love with her boyfriend, Tholomyès, who is thirty years old when she's just a teenager.
What Fantine doesn't realize is that Tholo doesn't exactly share her feelings. (It seems that, even though all three of her more experienced friends understand what's going on, they don't feel the need to clue her in.) As the narrator tells us, "It must be said that the three older ones were more experienced, more heedless, and more versed in the ways of the world than Fantine la Blonde, who was encountering her first illusion" (18.104.22.168). The suggestion here is that in 19th century France, women have to deal with a lot of "illusions" when it comes to men.
Of her entire friend group, Fantine is probably the least adventurous and the shyest. We read at one point that Fantine "was happiness itself; but she was also modesty. What the close observer might have perceived beneath the intoxication of youth, summer, and a love-affair, was an unconquerable reserve" (22.214.171.124). That's part of what makes her story so tragic. After all, she does everything right to protect herself—except get involved with a 30-year old player, we guess.
One day, Tholomyès disappears from Fantine's life completely, as if it were some kind of joke. Except—oops. Fantine "had given herself to Tholomyès as to a husband [translation: they'd had sex], and the poor girl had a child" (126.96.36.199).
Awesome. Now Fantine has to live as an unwed mother in 19th-century France. This is going to go great. From this point on in the book, Fantine's life is pretty much an uninterrupted stream of suffering, punctuated by little moments of pleasure in the fact that she's still young and beautiful: "But in the mornings, combing with a broken comb the hair that flowed like silk over her shoulders, she still had moments of happy vanity" (188.8.131.52).
Well, don't worry—even that's going to get taken away from her. Fantine works fifteen hours a day to support her child Cosette (who has gone off to live with the cruel Thénardier family). But eventually her shocking status as an unwed mother is discovered, she's fired from her job at Valjean's factory, and she ends up on the street.
She goes so deep into debt that she sells that pretty hair of hers and then eventually her front teeth to a travelling dentist. It's pretty gruesome: "She smiled as she said it, and the candle lighted her face. It was a bloodstained smile. There were flecks of blood at the corners of her mouth and a wide gap beneath her upper lip" (184.108.40.206). From selling parts of her body, she moves on to just plain old selling her body—and then it's all over. In the end, Fantine dies wishing that she could see her daughter one last time. But she never gets to have this wish fulfilled.
Fantine is like the counter-story to Valjean. They both break society's rules in order to survive—he steals some bread; she has a love affair—and, instead of getting help back on their feet, they're punished all out of proportion to the offense. Valjean manages to overcome his lapse, but only by taking on a new identity. Fantine, who has a kid as physical proof of her "sin," is out of luck. One mistake, and she's doomed forever, even though she works hard and tries to become respectable again.
If you feel really depressed by the time you're done reading about Fantine, you're supposed to. It's fairly clear that when Victor Hugo named this book "The Miserable Ones," he had people in Fantine in mind—and he wants us to think about what we can do to make sure that people like her don't slip through the cracks.