This note contains spoilers for the book.

A NASA astronaut is kidnapped by the moon, which is actually a partly-deactivated alien battleship, about 50,000 years old. Makes for a pretty zippy beginning. Battle Fleet exists to try to stop the recurring Achuultani incursions -- non-communicating aliens who destroy planets for fun and profit. Well, at least for fun; they don't seem to do anything profitable with them afterwards.

Like Hogan's Giants books, this is another "humans are not native to earth" story. There's a wave at reseeding after a previous Achuultani pass with common species to explain the similarity of the locally-evolved hominids to the imported ones (us).

Crazy bad guys; the people left on earth are all descended from mutineers, but some regretted it earlier than others. Through use of stasis, long life-spans, and in the case of the bad guys brain transplants, some of the original crew are still alive. No bridge officers, though; they were all killed off right away, and until Dahak gets Colin and wires him up, there aren't any in the system.

Meanwhile, the empire and Battle Fleet seem to have gone silent. And now the automated beacons show the next Achuultani incursion is beginning -- they'll reach Earth in two years. Seems hopeless -- how can we prepare a modern defense force in two years?

The book ends there; but there are two more books.