At the center of "Unity" is, of course, the parasite crisis; Bajor and DS9 are under seige and the Bajoran sector is under lockdown. Cardassian warships surround both planet and station, a communications blackout in effect. Gul Macet, previously seen clawing at his throat when a parasite launches itself at the Dukat-esque Cardassian, is in charge of the Cardassian contingent. This is the scene that Vaughn and the crew of the "Defiant" find themselves entering.
Without revealing anything too much, "Unity" gives us more of the inner turmoil Vaughn finds himself facing after shooting the Borg-Ruriko in sickbay, the anger Prynn holds for Vaughn, the deep-seeded Trill desire for privacy in all matters internal, and, of course, Kira's Attainder.
Ro and Quark are given a lot of face time, and not just towards their romance; each have their own demons to face: if Bajor joins the Federation, Ro fears she will be turned over to Starfleet for prosecution and Quark fears he will be forced to leave DS9 for more profitable pastures. Taran'atar bonds with Wex, the stowaway whom Jake Sisko encountered when fulfilling his part of the Avatar prophecy. While not everyone is who they appear, one person holds true to her nature; the former Kai, Opaka Sulan.
It is she who finds a Bajor in spiritual crisis, telling Vedek Yevir it is not the texts Kira uploaded that have caused the split in faith; it is the Attainder the Vedek Assembly placed upon the colonel that has lead to Bajor fractured. The former Kai does not step up to retake her mantle, instead opting to allow spiritual politics to play out as they will, acting instead as a spirtiual counselor to the lost Elias Vaughn. She exposes him to the Orb of Unity, allowing for Vaughn to come face to face with the man he first encountered in the Badlands after opening the Orb of Memory. His role is significant to the novel, and while he is not completely healed at the conclusion of "Unity," the centenarian commander is well on his way, thanks to the Prophets.
Ultimately, we discover the fate of Sisko and Kasidy, see the true face of evil (read: parasite), and relish the return of the O'Brien's to the fore. We leave the novel with an Andor in a peril of its own creation and an Andorian alone, a Bajor redefined and a faith still fractured, a son coming to terms with his father's place in the universe, and, as the cover shows, a former Bajoran freedom-fighter trying out a new uniform. Sometimes frightening, sometimes funny, sometimes bittersweet, and always breathtaking, "Unity" establishes Perry as the chancellor of the relaunch, leaving the reader satisfied yet wanting more.
Oh, I almost forgot; "somewhere on Bajor...a child long awaited is born."