Far-future space adventure set on a tidally stressed double planet connected by an alien-built space elevator. Space trouble-shooter Hans Rebka arrives in the Opal-Quake system to investigate why former whiz-kid administrator Max Perry has chosen to hide himself away on Opal. Watery opal and deserted Quake, Rebka learns, are connected by a space elevator system, built millions of years ago by the mysterious, long-vanished Builders (they left various semi-comprehensible artifacts dotted about the galaxy). Strangely--since opal-Quake is about to undergo a severe and dangerous Summertide or extreme tidal stress, something that happens only once every 350,000 years--a number of other VIPs abruptly show up: Darya Lang, an expert on Builder artifacts; double-brained galactic honcho Julius Graves; bad-guy Imperial representatives Louis Nenda and the insect-like alien Atvar H'sial. Why? Well, Lang hopes for an appearance by the Builders; Graves pursues a couple of child criminals; Nenda and H'sial intend to exploit whatever Lang learns about Builder technology, and will kill anyone who stands in their way. Everybody converges on Quake--the worst possible place to be as Summertide approaches. Lots of curious and fascinating alien gizmos, but otherwise rather nebulous and inconclusive: entertaining but not particularly memorable.