This is more a caper-story than Miles's deep personal development. It does have some spoilers for the earlier ones in that characters' mere presence will give away earlier plots, and the twists are more intriguing if you know the history behind them. (Obviously I can't judge how well the info-dump fills you in.)
Miles and Ekaterin are off on a late honeymoon, a year after the wedding. And on the way home, to return in plenty of time for the birth of the twins from uterine replicators, a message from Gregor intercepts them: Miles is to go off into quaddie space, and cool down a situation very close to war. A Barrayan military escort to a Komarran merchant fleet had gone in with guns to extract their men -- after an officer had vanished.
He meets an old friend there -- Bel, you may remember, had been interested in a quaddie they rescued from Jackson's Whole -- and discovers there is much more than meets the eye, even though the scene is complicated even when it hits. It involves an appeal for asylum, a trip to the ballet that Miles has to be persuaded into on the grounds of PR, a man with gills, a nasty bio weapon, someone with a lot of ambition, a second near-war, an immensely complicated search, synthetic blood, and much more.