This takes places between the opening of Dauntless and the ending of Invincible, but with minimal overlap of characters. (He resisted the notion of exclusively writing about John Geary.) Still, spoilers lie ahead -- more for The Lost Fleet than for its follow-on series.
It opens with conspiracy, as Drakon and Iceni move to overthrow the security forces, and get the space forces behind them, to wrest the Midway system out of the power of the Syndicate and into their own. Which means they succeed or die. . . but success, marked by the altering of names from the Syndicate's patterns to their own, does not mean they are out of problems.
So the rest of the novel deals with their wrestling with the problem of defending against the enigmas, the danger of the Syndicate returning, making the first stabs at relations with the neighboring systems, trying to work out how to alter the politics and society of Midway -- the Syndicate system had, after all, failed spectacularly -- without getting themselves deposed out of hand, and above all else, dealing their being two of them, after there had been four before their little coup, and both of them had climbed that high in a system composed of paranoia and ambition.
I'm not entirely sure that I believe that the Syndicate system would have lasted for as long as it did, when I view it in cold blood, but it kept disbelief suspended enough for the length of the book.