A conscious tribute to, and modern reinterpretation of, Edwin Abbott's famous 19th-century fantasy Flatland: science fiction, cast as non-fiction, exploring a theoretically possible but again hard-to-visualize two-dimensional universe. The scenario begins when Dewdney (a real-life computer-science prof) and his students construct, for didactic purposes, a computer program, ""2DWORLD,"" which attempts to simulate the characteristics of two-dimensional space--i.e., a place that has no thickness, like a mathematical circle. Presently they are surprised to discover an unprogrammed intruder, Yendred, a citizen of the ""real"" two-dimensional planet Arde--who resembles an intelligent four-armed hydra with, naturally, all his internal organs visible. So the story, such as it is, follows Yendred among his fellow-Ardeans as he travels from one side of Arde's only continent to the other in search of knowledge. Arde boasts many fantastic constructions--lifeforms, houses, boats, steam engines, computers--based on reputable research into the characteristics of two-dimensional atoms, gravity fields, electromagnetic waves, etc. And these phenomena, in turn, give rise to a number of fairly obvious philosophical questions. (E.g., are we being observed by imperceptible four-dimensional beings?) In all: an ingenious intellectual exercise--amusing, edifying, sometimes tedious--for those curious about the oddly circumscribed two-dimensional universe and its even odder mechanics.