I read this book right about the same time I was reading some theory on gaming, which was actually an interesting juxtaposition, since there are plenty of places in this text where gaming itself is discussed from both a philosophical and practical player-oriented point of view.
Jurnau Gurgeh lives in the Culture, a society in which there is no need, no poverty, no battles, no strife and whose occupants challenge themselves by playing games and creating art (including terraforming). Jurnau's friends include Yay, a terraformer, and Chamlis, a sentient machine (most machines in the Culture are sentient). This society has evolved to the point where people no longer need to work - what little work they do is in the form of hobbies more than necessity and therefore they have developed elaborate games.
But Jurnau is getting tired of games; he finds them unchallenging and after he allows another sentient machine, Mawhrin-Skel to help him win a difficult game against a newcomer, Mawhrin blackmails him into helping the drone get back into Contact, the interstellar branch of the Culture. In return, Contact challenge Gurgeh to a game unlike any other. It is a game played by another culture, one that bases its entire way of life, its merit-promotions, positions within government and society, on who is the best game player. The supreme ruler of this culture, Azad, is elected every game cycle (four years) as the person who wins the game of Azad.
Gurgeh agrees to the lightyears-long journey to play this game where he encounters a culture much like ours, with all the strife, jealousy and fighting that is absent in his own Culture. He performs much better than expected, and although according to the media, he has officially withdrawn, he actually beats the ruling monarch, Emperor Nicosar. In playing the game through its many stages and elimination rounds, he comes to realize that the game is actually a battle between his Culture and Azad, and Nicosar also understands this, attempting to assassinate Gurgeh and everyone watching the game by allowing a natural firestorm to overtake the castle in which they are playing the final round.
As I said, game theory is found dispersed throughout the text; at one point, the narrator tells us that the games a society plays reveals "so much about its ethos, its philosophy, its very soul". Makes me wonder what our games reveal about our souls?