On

Tighe is a young man who lives a precarious life on the face of an enormous wall. It's a wall so large that it's top can't be seen. Indeed, the wall extends in all directions further than anyone can see. People live on ledges on it, sometimes only a few feet from falling off the edge of the world.

Within his village community, Tighe is important, the son of a Prince. But in this post-apocalyptic world that doesn't signify much. People may trade in the relics of technology from a bygone age, but few understand it or appreciate the civilisation that has gone before. Tighe's grandfather, or Grandhe, is another big gun in the village hierarchy, the Priest. In a society where religion has replaced science the priest has all the answers, or at least he thinks he does.

Things become difficult for Tighe's family when they lose a goat, and even harder for Tighe himself when the inquisitive young man is accused of associating with heretics. But he doesn't start to understand the mystery of the Worldwall until he falls off the edge of it.

Tighe finds a new life in a much more populated part of the wall. But the society he has entered is preparing for war. The world of On is a harsh one, and the diminished population has become brutal, bordering on savage. People are intolerant and insular, and starvation or slavery are rife. Tighe has lived a sheltered life by the standards of his times, and the horrors and deceptions of war affect him badly. There are also nightmarish creatures that are said to prey on humans beyond the boundaries of human habitations.

On is a very unusual post-apocalyptic tale. The scenario seems to promise far more than could possibly be explained. The character of the Wizard, an ambiguous figure who seems by turns sinister and benevolent, appears to have some of the answers. However it's still quite unexpected when Adam Roberts comes through with a logical, if not exactly believable, explanation for this unusual world. The science behind it all is theoretical, and it does take some concentration to get your head round the concepts. But various different aspects of the way the world works are at least consistent, and for the scientifically-inclined there's a short appendix with equations.

This is an intriguing novel, a bizarre world that has come through a cataclysm and may yet not survive. The wall itself is a metaphor for the way humanity, and life itself, is only really teetering on a ledge, a hair's-breadth away from oblivion.

The main problem with this novel is a tendency to adopt a slow pace. Tighe is obviously not very well informed, since he had no formal education, and he can take what seems like a long time to figure some things out and to act on them. But on the whole this is a vivid and impressive novel which manages to be both highbrow and entertaining.