Parallel with Guardian in Beyond the Frontier series, and indeed, since I read that one, I can't review this one independently; there are situations that are viewed, albeit by different points of view, in both.
It opens with the arrival of the enigma fleet (the enigmas being an alien, of whom very little is known) in sufficient force to wipe out human settlement on Midway, which gives us much occasion for drama. Kommodor Marphissa positions such ships as they have to inflict the most damage when -- Black Jack's fleet arrives. The Midway forces join in, but the arrival is really what is key.
On the other hand, with his forces are two different kinds of ships, not only obviously not human, but also obviously not enigma. One great hulking ship being towed by Alliance ships, and six others, which in fact intervene on the human side against a problem no human can deal with.
Which throws them into other conundrums, enlivening problems that already existed. What information can they winkle out of the Alliance fleet about these aliens. How they can avoid looking too dependent on the Alliance -- just because they revolted against Syndic rule doesn't mean the populace has forgotten a century long war. Whether they should worry about a bright young officer who saved their repair base during the fighting -- ambition can be dangerous to them. Plus other things, like the probable ability to staff their unmanned navy if they take a long venture with several perils inherent in it and the question of whether to tamper with elections. Several assassination attempts. A liaison officer. Being escorted by Marines that you used to be at war with, and much more.
Definitely part of a series with a continuing story.