The Enterprise is searching for a Klingon battle cruiser that has been reported appearing at the extreme edge of sensor range, when it detects an immense craft, a city in space. Even more shockingly, its inhabitants turn out to be human.
The object turns out to be The Wanderer, a former space station positioned in the L5 Lagrange point, whose inhabitants first set it in orbit in the solar system, then aimed for deep space, following political disputes.
Their descendants, the current inhabitants of The Wanderer, have split into two factions. Those on the upper levels (nearer the center of the station, which has lower 'gravity') have a fairly pleasant existence. Those on the lower levels (who the others call 'demons'), however, must live on whatever scraps they can scrounge, under gravity as much as double Earth's, and without power. The two have been divided long enough that those on the upper levels consider it a religious matter, and refer to the leader of the other group as Satan.
There is no time for the Enterprise to try to carefully resolve this dispute, because before long the Wanderer will be inalterably on course for the Galactic Whirlpool–two black holes orbiting one another, cutting a destructive path light-years wide through the galaxy, releasing deadly radiation.
The contact team, led by Kevin Riley, collect one of the natives: Katholin Arwen ('Katwen'), a young lady who current works as a warrior. At length, they convince her that, contrary to the religious teachings of the Wanderer's captain, there are other spacecraft and other people in the universe, and that they must work together with the inhabitants of the lower levels to save her people.
Things don't go as smoothly as all that, but in the end, the day is saved.
The story is fine, although "people who have lived in a spaceship for so long they forgot planets exist" is a bit overdone by this point. Gerrold's writing is entertaining as usual, and he gives lots of interesting details to give character to the story. For example, he has Kirk repeat his name, 'Tiberius', as a way of reminding himself that he must be just, moral, compassionate–unlike the Roman emperor of that name.
Kevin Riley is at least as much the protagonist of this book as Kirk, which is great. Typically the lead characters in a Trek story will be Kirk and Spock, and perhaps some original character cooked up for the novel and as quickly disposed of, but Riley is a pre-existing character, and he'll be there after the book ends, too. Granted, other novels may not keep continuity with this, but it's nice at least to imagine that there are people experiencing things and growing, from book to book.