Can, I think, only be found in print in the Ciaphas Cain: Hero of the Imperium omnibus, which opens with the "Fight or Flight" where Cain first appeared, when Mitchell thought he was a one-story idea. (His eighth Ciaiphas Cain novel's been published.) That story doesn't have a perfect grip on the character. Here, however, he does.
It's told as an excerpt from Cain's own personal files, found after his death and edited by another character, who inserts quotations from other works to explain context Cain omits. And puts in her own explanatory footnotes. For instance, when Cain writes that he's left people to die and been untroubled by it, she notes that actually, he was nightmare prone. For if there is one thing certain about him, it's that his account is not entirely just to himself. He claims to be a total self-absorbed coward and jerk, but this is not entirely true. How far it is from the truth is a subject of much fannish debate, fanned by the author's refusal to pronounce on the matter.
So Commissar Ciaiphas Cain -- self-absorbed jerk and coward (albeit with self-interest enlightened enough to illuminate a small planet) or selfless hero with a good deal of sense and a perverse modest streak, or somewhere in between -- gets away from the headquarters where he's been assigned by his heroic reputation and back to a front-line regiment. Because, he explains, it was safer. At HQ, he was the guy to go to whenever you had a dangerous mission. He is welcomed by the officers, and the colonel even seems pleased to see him, which should have been a warning. Commissars are not popular people. He discovers that it consists of two regiments, amalgamated into one, officially, and actually on the verge of explosion. He has to cope with this before they get in sent into battle.
It involves an order to get a broom, a singer of all the old sentimental favorites, the assassination of a tau (that is, alien) ambassador, riots, a reorganization, a mission to the undercity's tunnels, diplomacy, and much more into an exciting tale.