A notably schizophrenic Heinlein--of which the first two thirds, despite frequent pauses to belabor the obvious and indulge in idiotic chat, is fairly enjoyable action-adventure. Aboard the space habitat Golden Rule, Dr. Richard Ames and Gwen Novak are plunged into a bewildering and dangerous series of events when an unexpected, possibly phony messenger is mysteriously murdered after delivering a peculiar message. The Golden Rule management inexplicably begins to harass the pair; so they flee Moon-wards--in a sabotaged spacecraft. And waiting for them on the Moon are plenty more bad guys, all bent on murder. What's going on? Well, first Gwen reveals that she's really a rejuvenated Hazel Stone! Her mission involves Mike (Mycroft Holmes IV), the now inactive intelligent computer from The Moon is a Harsh Mistress. Then, as a horde of baddies closes in, some interdimensional, time-traveling rescuers show up--led by Lazarus Long! At this point, alas, the plot walks through the wall--and the final third of the book is mostly explanations. There are many realities, and each can be changed; hence, a group of benevolent heroes--including the Empress Star and Rufo, Jubal Harshaw, and other favorite Heinlein characters--are striving to prevent two different sets of bad guys from ruinously meddling with reality. The good guys need computer Mike to help them make more accurate predictions--thus Hazel's mission; and Richard Ames is vital to the success of the operation--because the good guys' future history books say so! Curious, rather obsessive work in its forced, unnecessary parade of familiar figures from disparate novels; the effect is sometimes engrossing but just as often tedious, and the windup is disappointingly vague and undramatic. Still, Heinlein fans should be reasonably satisfied.