The human interest in Silverberg's novel comes from this dilemma. If the entire world is given over to provide for humanity's existence, what then does humanity do with itself? Inside the urbmons, fertility is revered to the point of neurosis. Producing even more inhabitants for this highly populated planet is the goal of every right-thinking person. It is to the greater glory of god (with a small 'g') for everyone to procreate as much as possible. Much of the "news" consumed by the populace is devoted to statistics regarding which building, which city within the building, which floor within the city, and which individuals on the floor are having more children than their neighbors.

There's also a tremendous societal interest in eliminating those emotional strains that make life in close quarters difficult for large numbers of people -- things like lust, adultery, jealousy, ambition, and materialism. The nominal equivalence of everyone's social status is, in fact, a complex balancing of the value of one's neighborhood, one's literal height within it (higher floors generally being higher classes of people), and one's ability to produce vast numbers of children.

The characters on which Silverberg focuses in the novel are struggling to maintain this balance. One character who appears in almost all the narratives is the tightrope-walking Seigmund Kluver, who begins the novel as a successful young intellectual destined to move to the very top. At fourteen, he has two children and a lovely wife. He is awakened by Charles where he is sleeping by the side of the bed, having had sex the previous night with Charles' wife. No one may deny anyone else access to themselves or their spouse for sex; this is one of the primary tenets on which the society is founded. Charles displays only a passing interest in Siegmund's visit to his home, but is jealous of Siegmund's fertile wife -- a feeling he suppresses. His own wife is sterile after only four children.

Siegmund reappears periodically throughout the novel, often "nightwalking," as Silverberg refers to this custom of wandering to others' homes for sex in the night. Though Silverberg begins by saying that anyone can do it, it quickly becomes clear that men do the actual nightwalking while women stay home and accept anyone who drops in. Though written in 1970 and obviously with much to say about the sexual revolution, the basic idea of the liberated woman seems to have escaped Mr. Silverberg.

The novel ends with the story of Siegmund himself, now fifteen years and five months old, and already staring at the end of what he can achieve within the urbmon society. Cursed with the desire to actually accomplish something in his life, he becomes depressed by his inability to understand the ruling class of the urbmon, whom, he comes to find out, use their status primarily for their own pleasure rather than to make life better for others. Distressed by this nasty dose of realism, alienated from his fellow citizens, Siegmund is the perfect tragedy of the World Within because he is a person who would be utterly fulfilled by life as we know it: competitive, luxurious, individualistic.