This book is out of order with the other books (it would kind of have to be after "Cain's Last Stand" taking place technically after Ciaphas retires) and that was a little bit weird for me because there were certain things that the readers knew, but Cain himself didn't know as the action was taking place, but thankfully, as with all the Cain books, we have the footnotes written by Amberly Vail.
Cain has been assigned as an Liason officer to the Reclaimers space marines chapter. I guess he figured with the biggest, baddest bad asses in the galaxy surrounding him, he should be pretty safe.
Silly Caiphas, you should have known better than that. Cain deals almost exclusively with genestealers in this book, and while we've seen him deal with Tyranids before, seeing him tool around a space hulk with terminators was kind of different.