Note: In this article, "Moriarty" always refers to Jim Moriarty, the Sherlock character, and not Conan Doyle's Professor Moriarty.
At least three plot points in "A Scandal in Belgravia" don't work. There may well be more, but these three are enough to kill the plot and, therefore, the episode.
The photos on the cell phone do not work as Irene's bait. Even if Irene disabled the upload/download link, the contents are not "provably unique." She'd never just hand over her precious phone to someone else to publish the images (after all, she wants to keep the rest of the data she claims is on the phone). So to use the images for blackmail, she must have worked out some way to get them off the phone, in which case there could be copies. And we're just supposed to take her oh-so-reliable word that there aren't any? (Sherlock, Mycroft, Moriarty, and Moffat should all have thought of that.) Therefore, Sherlock and company must assume either that she does have copies (in which case, securing the phone is pointless) or that blackmail is impossible (ditto). OK, so maybe the members of the British royal family aren't smart enough to work that out, but they would've gone to Harry (which they did) and he would've gone to Mycroft (which he did) and Mycroft would've laughed in Irene's face. (It would have been very reminiscent of Holmes' reaction to Miss Adler's threats until the King of Bohemia told Holmes that both he [the king] and Miss Adler were in the photograph she had.) And that's not even to get into the whole ability to fake photos, which becomes harder to detect every day. To give Irene something truly unique, we'd have to go low tech, and it would have to be valuable in its own right (like an original work by a great artist is). For anything born digital or that can be made digital, there's the copy problem. For anything stored only in her head, two specimens like Mycroft and Sherlock would be able to extract it from her (as Sherlock in fact does). Irene isn't bright enough to realize that, of course (since she in fact doesn't), but if she were a worthy successor to Miss Adler, she certainly would have. That, of course, is another article.
I grant, I cannot myself think of something to replace the photos that Irene might have had that would get Mycroft's attention. The photos are out, though.
Irene's hypodermic attack on Sherlock is just plain silly. Sherlock, knowing that he carries something that Irene has just said she would rather die than let him take, lets her near enough to stab him with a hypodermic syringe (and in the opposite arm, no less)? How dumb would he have to be to do that? Anderson wouldn't have fallen for it, no matter how infatuated he was! Her success lowers the IQ of the entire audience. That whole section of the episode is just plain wrong and should be stricken. It was therefore unnecessarily rude to upstage Benedict Cumberbatch with a folding bed, and Mr. Moffat needed to prove Irene was truly smart some other way than the drugged sequence where she solves the dead hiker mystery.
Worst of all, the Coventry text string predicament, on which the whole episode depends, does not work. First, Sherlock solves any puzzle for free, just to show off. It is one of the defining attributes of his character, as he himself recognizes in the very next episode: "Of course [I am showing off]. I am a show-off. That's what we do" ("The Hounds of Baskerville"). There was no need for Irene to make him "feel special," as Mycroft asserts. There was no need for Irene/Moriarty to spend six months softening Sherlock up. All Irene had to do was hand him the code and tell him she didn't know what it was (she could've even saved her "I'm scared" act for a more necessary occasion), and eight seconds later, she would've had what she and Moriarty wanted. She wouldn't even have had to reveal herself that much. She could've sent Sherlock the text string anonymously, and he would've sent her the answer. (He might then have become curious regarding who sent him the puzzle and have tracked her down, but that would have been an appropriate moment for her "I'm scared" act.) Sherlock needs peopleeach and every person!to know that he can figure anything out. Moriarty's hostages in "The Great Game" were actually superfluous, and Moriarty must've included them just because he enjoys that kind of thing.
Second, if Sherlock can crack that code in eight seconds, then Moriarty could've done it in six to ten seconds. Why is he having Irene go through Sherlock? What on earth does he gain? Embarassing Sherlock? Forcing Mycroft to report to his "masters" (if he has any) that his younger brother is his biggest security risk? Surely Mycroft knows that Sherlock is his biggest security risk; remember, Mycroft is the smartest thing we've met yet, even more so than Sherlock, and he's known Sherlock since there was a Sherlock. He must assume that someday someone will send Sherlock top-secret information that he will happily decrypt and then even Mycroft doesn't know what Sherlock will do with it. And being Mycroft, he has prepared for this eventuality. Moriarty must surely know all that, tooyou don't have to be a Mycroft to figure it out; surely a Moffat would do!so the investment of time and resources is much too large for the pay-off of having only Sherlock, Mycroft, Moriarty, and Irene know that Sherlock has, for a very brief period, egg on his face.
In short, Moriarty is smart enough to handle Irene's matter on his own, and he gains nothing by involving Sherlock, not even much of a chuckle. He could've just figured out what the text string was himself (six to ten seconds), told his terrorist customers what was up (ten minutes, tops), collected his fee, and (since Irene was so very stupid!) not shared the proceeds with her. Then he could've chuckled about how he made an end run (or whatever the soccer or rugby equivalent is) around the Holmes brothers, enriched himself, and humiliated and defrauded that upstart Adler, who actually thought she was on his and the Holmeses' levelmuch less effort and risk and much better profits in all senses, especially when getting Irene her big haul is nearly certain to fail.
The first two plot breakagesthe photos on the phone and the hypodermic attack on Sherlockwould be enough to seriously damage the plot of the episode. The Coventry text string is fatal, however, and it does have the other two incidents to keep it company. Yes, no matter how many scantily (or non)clad women, sexual inneuendos, and humorous incidents appear, "A Scandal in Belgravia" just doesn't have a functioning plot, and without that an episode cannot even be called acceptable, let alone award-worthy. This point alone damns it as a bad episode, and the broken plot does have all the items I will mention in my other articles to keep it company.
Please, please, take back the Edgar and give it to some episode that actually hangs together. "The Reichenbach Fall," although it has a few minor problems, springs to mind.