Most readers did not realize in 1980 (when it was originally published) that the novel had a sister book, written in 1977, that was never published. That book is finally being published under the title . Both novels deal with parallel universes, share the same main characters and have the same first one-third of the book. However, from that point on (after they make a jump to a parallel universe) the novels diverge completely. And here is where the second part of the experiment comes in. While continues the adventure in a very customary Heinlein manner, reminiscent of his earlier works, becomes something very different. On surface, the book is about two men and two women who are attacked by aliens and then embark on roller coaster ride of an adventure through a myriad of universes. But as Jack Kirwan wrote in The National Review, “describing thus is like saying Moby Dick is about a one-legged guy trying to catch a fish.” is a homage to science fiction, to his friends and to characters used in other books, also serving as a parody and a lesson to anyone willing to listen, in a way only Robert A. Heinlein could have presented it.