To their surprise, they find their artificial 2D universe has somehow accidentally become a means of communication with an actual 2D world: Arde.

They make a sort of "telepathic" contact with "YNDRD", referred to by the students as Yendred, a highly philosophical Ardean, as he begins a journey across the western half, Punizla, of the single continent Ajem Kollosh to learn more about the spiritual beliefs of the people of the East, Vanizla.

For example, all houses are underground, so as not to be demolished by the periodic 2D rivers; nails are useless for attaching two objects, so tape and glue are used instead; most Ardean creatures cannot have deuterostomic digestive tracts since they would split into two; even games such as Go have one-dimensional Alak analogues.

This was reviewed by Martin Gardner in his July 1980 "Mathematical Games" column in Scientific American, and shortly after this, all copies of the monograph were sold out.

[1] Dave Langford reviewed The Planiverse for White Dwarf #55, and stated that "This delightful book will be inspiring 2D game scenarios any second now.