The long-awaited sequel to Vinge's enormous The Snow Queen (1980), an interstellar tug-of-war between the far-from-benevolent Hegemony and the backward-but-indispensable planet Tiamat. It is now Summer on Tiamat; the Hegemony has withdrawn, leaving the planet in the hands of the Snow Queen's clone, Moon. Numerous--too numerous--subplots get underway. Moon's former lover, BZ Gundhalinu, will be sent to World's End, where a wrecked Old Empire ship has spilled semi-sentient stardrive plasma; if Gundhalinu can control the plasma, faster-than-light travel will again be possible, ending Tiamat's periodic isolation. Elsewhere, Reede Kullervo, a researcher with a rebuilt brain, addicted to his own supercharging designer drug, will be ordered by the leader of the supercriminal Brotherhood to seek the immortality elixir whose only source is Tiamat. Meanwhile, Moon struggles to control Tiamat's rebellious factions, knowing that the planet's intelligent sea-dwelling ``mers'' are the source of the elixir, and that the ancient computer that links the galaxy's clairvoyant sibyls in an information network lies buried under Tiamat's chief city, Carbuncle; she dares not permit the Hegemony to control either the sibyl network or the elixir. Overcomplicated, as usual, with too many flabby patches, but much more rigorous and logical than its predecessor. Too, the pretentious allusions have been banished in favor of a welcome infusion of new ideas: all in all, an absorbing and satisfying performance.