Edwin Abbott wrote Flatlands before Einstein formulated his theory of relativity, so the concepts of alternate universes and dimensions beyond the third existed only in very abstract theory. Considering that, it's a fairly impressive book on a purely theoretical level.
The plot of the book, such as it is, mostly takes the form of a travelogue (so listen up, Nathan) of a two-dimensional world. The protagonist, A. Square, gives us a tour of a land where women (who are just lines) are required to sway side to side when they walk, and where judging people by sight is considered a greater virtue than getting to know (or “feeling”) them. Most importantly, it's a world that can't even conceive of a third dimension, and even the suggestion of such is a cause for death.
In the final section of the book, A. Square's world is turned upside down when he is visited by Sphere, a creature from the third dimension who also introduces Square to Pointland, the land of one dimension. The book is a pretty clever satire, and it fits a lot of content into its scant 100 pages. It would probably take less time to read than this review, and would be more rewarding.