For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
Linda's happy little life ends at six years old, when she (1) realizes that she's a slave and then, a few years later, (2) is sent to work for the Flint family. Dr. and Mrs. Flint are apparently in competition to see who is the bigger sociopath, and they torment Linda emotionally, physically, and—in Dr. Flint's case—sexually.
Act II
Eventually, Linda's had it and runs away. But not very far. By this point, she's had two children by another (white) man, and she can't bear to leave them. Her solution, which we can't entirely get behind, is to hide in the leaky, rat-infested crawlspace above her grandmother's shed. For seven years.
Act III
Linda’s friend Peter finds a ship's captain to carry Linda to Philadelphia. The North is not as awesome as Linda had hoped, but it's still pretty great, especially once she manages to reunite with her children. Eventually, she finds work, friendship, and freedom: almost, but not quite, everything she wants.