In essence, Palimpsest is a book reading a bit like poetry about a mythical sexually-transmitted city (yeah, you read it right!), and how four ordinary strangers happen upon, and are therefore affected by, it. Or, well, technically, how it happens upon them. You see, everyone who gains access to Palimpsest has tattooed, in some part of their body, a map representing an area of the city they can visit without help. To visit other areas, however, one must have sex with people who have the place one wishes to mapped on their skin. It doesn't help matters that the city is incredibly addicting, often being described as more real than the waking world, and far more beautiful. Thus, the city often indirectly (and its citizens often directly) wrecks the lives of all those who find it.
