In Hogan's intriguing fifth SF novel in the series that began with Inherit the Stars (1978), Earth has reestablished contact with the Ganymeans, an alien race that manipulated proto-humans into homo sapiens on Minerva, a planet that once occupied the region of the present asteroid belt. After the Ganymeans migrated to the Giants' Star 20 light-years from Earth, a war on Minerva caused by intelligences from an alternate reality-one of an infinite number suggested by the Multiverse hypothesis-led to the planet's destruction. Now, several decades into the 21st century, people on Earth have developed a means of exploring these realities, including one in which Minerva still exists, and mount a rescue mission to prevent the war on Minerva. While the need to establish the backstory slows the book's first half, Hogan does an excellent job of extrapolating the science from current theories of quantum physics. The second half moves briskly and logically to a satisfying climax, though the villain is straight out of James Bond. Readers who like their science hard will find this one a diamond.