The most interesting thing about this episode of "Mannix" is that it was directed by Fernando Lamas. Yes, the actor Fernando Lamas who swam with Esther Williams in "Dangerous When Wet" and appeared in many more MGM films in the 1950s.
As far as the plot goes, it's not that interesting--mostly because I've seen this same plot on other shows (such as "Hawaii Five-O". It begins with a riot having occurred on death row and the prisoners have taken a guard captive. Now, out of the blue, these prisoners have demanded that Mannix come and see them or the guard dies! When Mannix arrives, he realizes he knows none of these prisoners--but he does know the guard. Apparently, the guard told the prisoners that Mannix was a great private detective--and the riot occurred because they want a detective to investigate one of the death row inmate's cases, as they are apparently convinced the guy was framed. While at first this seems very unlikely, it does make sense, as the ringleader thinks that proving an innocent man is about to be executed will possibly lead to changes in the law-- and possibly saving the rest of them this fate.
While this idea is interesting if not original, the show also has a few sloppy moments. The disclosure at the very end in front of the prisoners seemed contrived. Additionally, while Mannix may not have found 100% definitive proof that there was a frame, there were MANY things that made it seem likely--so much so that I cannot see why they didn't get the governor or courts to at least delay the scheduled execution. You'd think folks beating the crap out of Mannix and telling him to NOT investigate would at least send up a few red flags! And, the private investigator who admits he deliberately botched the case...that also could have resulted in at least a delay. Oh well....overall it's a very watchable episode--just not the most logical.