This is the second half of the Enterprise's escapades during the Dominion War, not told or shown during Deep Space Nine's run. Basically, Picard hears a rumor that the Dominion is building a wormhole in Cardassian space and with the help of Ro, who's Maquis friends have been displaced and killed by the Cardassians and their Dominion allies, they infiltrate Cardassian space to verify the rumor and, if they can, destroy the wormhole before it can be activated. In the first book, the discover that the rumor is true and manage to help save some of the Federation prisoners being forced to work on completing the wormhole as slave labor. They've retreated to the Badlands in order to figure out how to either disable or destroy the wormhole before it can be completed.
And that's where this second book picks up. Picard and crew begin formulating a plan, but sabotage forces them to consider the possibility that they have a traitor on board. It also forces them to fight for survival in the Badlands, where they discover some long abandoned ships. Once they've retrieved food and water (and discovered the secret of the ships) they turn their attention back to the wormhole and come up with a plan to at least hurt the progress on building the wormhole, in the hopes of giving the Federation enough time to destroy it before it can be used. The only problem is, it's a suicide mission, since the traitor in their midst has destroyed their ability to contact the Enterprise to ask for help.
This was a good follow-through with what was built up in the first book. In fact, I enjoyed this book more than the first, which felt kind of rambling in places. This one was much more focused, and adding the traitor and the fight to survive even before they can consider how to disable the wormhole was well done. My only real complaint was that the ship's ability to get as close as they got to the wormhole pushed the wall of believability. And then that wall crumbled when the traitor notifies the Dominion of their position and only one Dominion ship is sent to find them. Once that ship is destroyed, there isn't a second attempt made to locate them and plans progress after that without the Dominion continuing the search or even reacting to the destruction of the one ship. So there were some credibility problems with the main plot toward the end of the book, when the action kicks into high gear. But this is Star Trek, and there are often parts of plots where the bad guys should have reacted in more intelligent ways, so most readers will forgive all of this and just enjoy the story. And I did enjoy the story. The main action at the end, once their final plans are put into motion, is great.
So, an enjoyable addition to the Star Trek universe and a great additional facet to the Dominion War we saw being played on out Deep Space Nine.