Sorry to repeat and rephrase myself but I don't want to try to edit a post that already has answers and I have since slept on it. Does it make sense to think that: Sherlock's first reaction to the crisis (as related to him by Mycroft on the phone) is to investigate how Jim could possibly be alive (rather than, say, how to trace the perpetrator whoever they are), BECAUSE Mycroft's call contained the information that Jim is alive, presented as a fact beyond all doubt, certain knowledge. And that's new information to Sherlock, so Mycroft has known but kept it from him.

Is that reason enough to think Jim spent those years under arrest and has only now escaped? Because otherwise Mycroft couldn't know for sure?

Why did Mycroft keep Sherlock in the dark?

What would make it impossible or unlikely that Jim has simply been in hiding? That Mycroft could not tell Sherlock on the phone "look, I KNOW that he is alive"? Anything else?

Why would Sherlock's conclusion from his MP experiment be that Jim is certainly dead? The bride was, but there were the additional circumstances of her body in the morgue etc. So IS Sherlock lying as he says that? Does he mean to confuse Mary? Is he lying to John?

How likely is it that Jim's next move, which Sherlock indicates he jnows what it is, is simply "coming after Sherlock", just as the Victorian Moriarty apparently did?

Even with all that, why would Sherlock's first response to the crisis be "I must know how he survived" instead of something more practical along the lines of "how do I stop him" or "alive or not, is that really him behind the crisis" or "so, does the mysterious keycode exist after all" or whatever? Is that a necessary first step, or does he just need that for his own relative peace of mind maybe?

Good one.

I may have misunderstood some of the comments above, but it sounds like you folks are leaning a bit towards my pet theory, that Moriarty faked it and either escaped or was arrested on the rooftop as soon as Sherlock jumped (thereby explaining how Sherlock planned to prevent Moriarty from looking down too soon and seeing that he'd been tricked by Sherlock.)
 
I really love some of the new ideas presented above that fit in with this line of reasoning.  For example:
 

tehanu wrote:

TAB does change things? Before it, Sherlock thought Jim was dead, and only now thinks he is alive? I think that "of course he's dead" line is actually a lie for Mary's benefit.

Wow, that seems quite plausible.  Good one!  I hadn’t thought of that possibility at all.
 

Liberty wrote:

Why doesn't Sherlock just assume that the video we see was pre-recorded?  What's the mystery?  If he actually saw Moriarty's unsurviveable head wound, then why does he need to question whether he's dead or not?  
 
It's not as if TAB actually answers that question (Moriarty is dead, but his supporters could have set things up to look like he's back - really?   Isn't that the just the first thing Sherlock would think?).

Exactly!  Sherlock seems to doubt his prior conviction that Moriarty is dead.
 

SusiGo wrote:

This is indeed an interesting question. My only spontaneous answer would be: because Sherlock thinks that Moriarty, if alive, would wish to continue their game in exactly this way. If Jim wants to play with him, he needs Sherlock alive, not on a death mission. This would be no fun. 

The video probably reminds Sherlock of the video set up for him in the cab.

 
Ouch!  Why didn’t I think of that?  The playful way Moriarty competes for the coveted title of Smartest Guy in the World is obviously what’s driving Jim to come back — not some act of revenge against Mycroft because he locked him up for two years (if that turns out to be what happened).
 

tehanu wrote:

Why would Sherlock's conclusion from his MP experiment be that Jim is certainly dead? The bride was, but there were the additional circumstances of her body in the morgue etc. So IS Sherlock lying as he says that? Does he mean to confuse Mary? Is he lying to John?
 
Even with all that, why would Sherlock's first response to the crisis be "I must know how he survived" . . .  Is that a necessary first step, or does he just need that for his own relative peace of mind maybe?

That’s the way it struck me, too. Remember the unusual way Sherlock acted in The Hound of Baskerville, when he thought he saw something (the hound from hell) that his mind could neither explain nor except?  And part of his hallucinations were terrifying visions of facing Moriarty, which caused him to scream things like, “No, no! How can you be alive?”
 
I think the situation in TAB is identical. Sherlock is incapable of just saying, “Well, he faked his suicide and he’s still alive, but I’ll worry about how he did that later.  For now, let’s just hunt that lunatic down.”
 
TAB spent an hour and a half showing us (and Sherlock) that a person who seemed to be dead had actually fooled everybody.  Then Sherlock seems to refute the possibility that Moriarty had done exactly the same thing when he stated flatly that Jim was dead.
 
And yet he tempers that statement with “I didn’t say he was alive, I said he was back . . . and I know exactly what he’s going to do next.”
 
So, I still think we’re being prepared for the return of Moriarty.
 
 

Last edited by Bruce Cook (January 15, 2016 4:10 pm)

Agreed. His legacy lives on but he is 'brown bread'.

Sherlock appears to be convinced Moriarty is dead. But there's always something, isn't there? 

I have to agree with "Jawn" that Moriarty is dead - as Sherlock seems to indicate, first, by making it clear that he never said Moriarty was alive, only that he was back, and then by flat-out saying that there is no question that Moriarty is dead. However, I think the statement about "knowing exactly what he's going to do next" is another one of those moments when Gatiss and Moffat imply one thing but mean another - and they tell us exactly what they mean without actually overtly telling us.

I think the real meaning of the statement is shown in the body language, not in the words. Notice that when we hear the words, "Moriarty is dead, no question. But more importantly, I know exactly what HE'S going to do next," Sherlock glances away - towards the plane - when saying the word "he." And who is still on that plane (or perhaps, just climbing out of the plane to get into his car)? Mycroft, of course. I think what Sherlock is saying, without saying so, is "Moriarty is dead, so there is no way he has come back to wreak havoc on England - which means Mycroft engineered all of this to have an excuse to bring me back, and now that I know Mycroft is behind it all, I also know exactly what my brother is going to do next."

And what is Mycroft going to do next? Sherlock tells us that too, in his parting shot to Mycroft as he gets off the plane and heads to the car:  "What are you still doing here? Shouldn't you be off getting me a pardon or something, like a proper big brother?"

My theory is that Mycroft used a combination of news photos from Jim's trial and audio-visual footage taken of Jim during the two-month period that he was being interrogated by Mycroft and Co. (and while Mycroft was feeding him a combination of tiny truths about Sherlock and some well-crafted BS), and spliced them together to create the GIF that was broadcast throughout the country. It was enough to scare Lady Smallwood and the various ministers / MPs into recalling Sherlock, and enough to drive home Mycroft's earlier point (at end of HLV) that England needs a knife it can yield with precision. Mycroft will use the leverage he's created to secure a pardon for Sherlock - and perhaps give Sherlock a more official status with the entire police force (since we saw the Commissioner's [actually correct] opinion that Sherlock should not have had such free rein to see 'classified' information and visit crime scenes, given his status as an "amateur" consultant). Once that's done, there will be some revelation that Moriarty's really dead, with a fake cover story that he was killed by rivals or by people in his inner circle during a power struggle, and the stunt that brought Sherlock back from exile will somehow be attributed to one of Moriarty's henchmen who were hoping to convince the world that he was still alive and therefore could be blamed for whatever havoc the henchmen were planning to wreak... and then the story will move on to whatever else Moffat and Gattis have up their sleeves for Seasons 4 and 5. (I suspect they may bring in another Moran - the sniper one from ACD canon, who will turn out to be Moriarty's second-in-command, and a brother or cousin of the wussy Cabinet Minister / spy / would-be bomber Lord Moran from TEH; and if they go with canon, they may "off" Mary by making her a casualty of Moran or another henchman, perhaps as part of the quest to rebuild Moriarty's network and/or avenge his dead boss. If they do kill Mary off, I imagine that Baby Watson will be collateral damage somehow and not survive, either - since without Mary in the background to care for the child, that means John is going to have to find a nanny or become a stay at home dad, but in either case, as the child's sole surviving parent, he's likely to give up dangerous pursuits that could leave the kid an orphan, which would put an end to his adventures with Sherlock. So, adieu Mary and Baby Watson, hello heartbroken and depressed John moving back into his old digs in Baker Street.)

As for the gaping plot hole that was there throughout Season 3 - namely, the fact that both Moriarty and Rich Brook pretty much disappeared from the scene after Sherlock died, and no one seemed to wonder why - I think that other than Sherlock and Mycroft, no one knew about the suicide on the roof, so the assumption would be that after the "Rich Brook" expose and the news of Sherlock's suicide hit the papers, people would take the "Moriarty was a fake" story at face value and since he was only a stage creation, no one would miss him if he disappeared. One would think that perhaps Rich Brook's sudden disappearance would look suspicious to someone, but again, given the nature of Kitty Riley's story - in which Brook admitted that he DID commit crimes, because Sherlock paid him to do so - it might not be a huge surprise if 'Brook' apparently packed up and went underground to avoid arrest and prosecution for his role in the hoax.

As for what happened to Moriarty's body, I'm pretty sure that during the post-jump stage-dressing bit, Sherlock would have told one of those who were in on things that there was a body on the roof and would have told someone to relay the message to Mycroft, who then saw to it that the body and the mess were cleared away by the time the (real) police had arrived on scene. It would have had to be done quickly since the police would definitely have gone up on the roof to examine the location from which Sherlock had jumped, and even someone as dim as Anderson would have noticed if there was another dead body and large pools of blood and brain matter up there. So, Mycroft's team somehow disposed of the body (or preserved it for use at a later date, if needed), the police (including Lestrade) were none the wiser, Anderson's theory (as told to Lestrade at beginning of TEH) was just that (yet another far-fetched idea among dozens of very elaborate conspiracy theories he'd been pitching for the last two years), and as far as everyone else was concerned, Moriarty and/or Rich Brook had made a run for it and so good riddance to bad rubbish - which would explain why everyone (except John and Mary) about pee'd themselves for real when the Moriarty GIF appeared on screens all over England.

In short, Moriarty's dead (but may be back in the mind palace or in flashback scenes), Mycroft used Moriarty's image to manipulate the Government into pardoning Sherlock, and we'll be back to the usual business of making deductions and solving crimes without any pesky murder charges hanging over our hero's head. [My theory is that Magnussen's death was reported in the press either as an accident or heart attack/stroke at home, or as the result of a botched home invasion attempt by person(s) unknown, so only a small handful of people in England will ever know the truth of what really happened - since if everyone knew that Sherlock had killed him to protect John and Mary from blackmail, the police would not be likely to let a known murderer work on cases with them, nor would Sherlock get many private clients, either. And both of those realities kind of put a damper on future story telling...]

That's a fine piece of work, dottoressa, and it does sound very much what the producers would do.  I'm still hoping, however, that what transpires in January confirms my belief that a true Sherlock-level genius like Moriarty would not kill him because he believed that would force Sherlock to jump.

After all, it didn't work, right?  So he died for nothing.  Dumb plan for a genius, eh?

What DOES seem smart (and interesting story-wise, as I've said to death on this fine forum — and please forgive me), is for both men to enact clever suicide hoaxes, and both men fall for each others ruse at the time they were conducted.   Sherlock thought Moriarty was dead, so he jumped, and Moriarty wasn't allowed to see that Sherlock landed in a fireman's net below, because Mycroft's men rushed out and apprehended him shortly after Sherlock went off the ledge.

The Big Reveal in January would then be when Mycroft admits that Moriarty had been in custody during the entire time Sherlock dismantled Moriarty's network, but he recently escaped.  I also love the idea that while Moriarty was in custody, he was kept from knowing that Sherlock was still alive.

Wouldn't you love to see the scene in which Mycroft brings Sherlock up to date on all this?  It appeals to me so much more than just having Moriarty dead because he wasn't as smart as he thought it was.

Mycroft would look like this . . . .



 . . . and Sherlock would look like this.  :D