A friend of mine introduced me to Flatland through the movie, I watched a bit of it and was instantly intrigued.
The first half of the book describes the world of Flatland, from the rigid hierarchy of their society to the weather. Abbott creates a fictional world that brilliantly satires the society of his time. Shapes with more sides are more respected members of society, with the top ranking social class being circles and women being simple lines and irregular shapes are either destroyed or used for experimentation.
The second half of the book describes A Square's visits to other dimensional lands. First, to Lineland - where he attempts to explain to the one-dimensional inhabitants that he is indeed two-dimensional, to no avail. Then, to Spaceland - where he becomes the unwilling believer as a Sphere tries to get him to open his mind. This section, I think, is Abbott's call to others to look beyond the constraints of what we know to be true into the unknown.
It was not an extremely easy read for me. Not only are some passages a bit technical, but it's written in that elegant Victorian style that isn't always the easiest to read.
All in all, a very clever satirical take on Victorian society.