It's the distant future, and a star-spanning civilization, populated by multiple human-descended species, known as "Prime" species, as well as a number of genuinely alien intelligent species.
Centauri pilot Sethran Kada unexpectedly drops out of subspace into the wrong sector of space, and quickly discovers he has an unexpected stowaway. The stowaway is a being from subspace, a member of a newly evolving species native to subspace, and has stowed away in his brain. searching through his data files, the being constructs an appearance and a personality, and decides she's female and names herself Khoe.
She wants his help stopping the kidnapping of her people into realspace, and especially his help in recovering the being she calls the Alpha--an individual vital to her species' continued existence.
They are both in for a wild ride.
I like the characters. I like the action. I like the gritty, lived-in feel of the universe. The story is a lot of fun.
But it's only fair to make clear that while one major plotline is resolved, this book definitely ends on a cliffhanger. Subsequent volumes are available, but be aware that you'll be wanting them.
There are also some oddities in terminology which seem hard to account for. Spaceships are often referred to as "planes," and the military is called the Air Command. This just doesn't fit. If you are very sensitive to that kind of inconsistency grates for you, be aware before you start reading.
All in all, though, enjoyable reading.
I believe I received this book for free. I'm reviewing it voluntarily.