I read this many years ago and I was hoping that it would be just as good the second time around. Alas, nostalgia often causes one to have fonder memories than reality dictates. Nevertheless, I really do think it is a really good book. Humanity has finally explored the solar system and started on the beginnings of interstellar travel. As the exploration proceeds, though, it becomes apparent that people from Earth have already been in the places that are being explored and that beings from elsewhere have too. Was there an interstellar war sometime in prehistory that had wiped out the accomplishments of the inhabitants of Earth so that they had to start all over again? When the remnants of an Earth colony is found orbiting Sirius it appears so. It also appears that there is at least one weapon left over in the solar system that is causing trouble for the explorers from Earth. It's too bad that they take so long to figure out that it actually is causing trouble for them.