Mark, masquerading as Miles, raids Jackson's Whole with the aid of the Derendarii Mercenaries to free the clones that will otherwise be used to prolong the lives of the obscenely rich. Of course, not being Miles, Mark doesn't quite carry it off and, in the course of rescuing his brother and his troops, Miles is killed. Preserved in cryochamber all would be well, except that the mercenary who hides the chamber is also killed, before he can reveal the location to anyone, and without any chance of cryopreservation himself. A fearful Mark meets his genetic parents, who unconditionally accept him, but is wracked with guilt about Miles and returns to Jackson's Whole to try to find him.
I really like the character of Mark and enjoyed reading a little more about him. While it's clear (given there are more books in the series which is, after all, called "the Miles Vorkosigan saga") Miles survives, Bujold keeps the suspense wire tight, and the complex interplay between characters is as involving and interesting as ever. The whole Mark, Cordelia and Aral dynamic is fascinating, particularly as it's overshadowed by their anxiety about Miles.
I know I haven't mentioned this before, and the details are always subtle and discreet, but Miles really gets around - a number of past lovers make an appearance in Mirror Dance and even while fighting for his life Miles manages to add another to his string. In some ways Miles is like a shorter, more flawed, less arrogant, more human James Bond! - Alex