The final installment of the British sf grandmaster's sweeping, enormously impressive trilogy about Helliconia, a planet with two suns--where the notably severe seasons last for centuries. Now, as the long, frigid winter rapidly approaches, Helliconian lifeforms are undergoing an abrupt transition from heat to cold adaptation; the phagors, shaggy, minotaur-like, sapient rivals of humanity, are emerging from their summer retreat to challenge human supremacy. Ironically, the dangerous phagors are essential to human survival: they host ticks which carry the Fat Death, a virus that transforms the hot-weather humans into barrel-shaped, cold-resistant types. But the harsh, unseen Oligarch of Sibornal, pursuing personal power instead of species survival, has decreed that all phagors must be killed and the Fat Death kept as bay by any means. And meanwhile, in didactic and rather distracting interludes, we learn that the inhabitants of the orbiting satellite Avernus (which televises the proceedings back to vastly distant Earth) have degenerated into a grotesque barbarism--as Earth itself, devastated by nuclear war, gives rise to new orders of being: the enigmatic, polyhedral geonauts; and a new, highly empathic, non-violent strain of humanity. Aldiss has taken particular pains to make this wrap-up entry more accessible--adding careful explanations, cutting down on subplots and jargon. So, despite the pedagogic baggage (grim philosophizing) and uneven drama, this is another brilliantly inventive, splendidly thought-out effort: the superior finale to an awesome and fascinating spectacle.