You're not gonna see your friend again, Sherlock.
Hah, Redbeard... Do you want to play?
You... Maybe you... Maybe me?
God save us, the Dead Bride is real!
The first episode of "Sherlock" begins with the war scenes in Afghanistan. We don't understand what is happening until the plot goes further and we can notice that it was a nightmare of the first character that we see - John Watson. It represents him as a post-combat.
The name is Sherlock Holmes and the address is 221b Baker Street.
He's always been like that...
In the third episode of the season four we can see the flashbacks connected with Sherlock's past. We see his childhood and family for the first time, also this flashback tells us about Eurus, Sherlock's sister, who was somehow connected with her brother's dog - Redbeard.
Viktor... You killed my best friend.
You really don't understand, Sherlock, do you? Redbeard...
Basically, the whole Christmas episode of Sherlock is one big flashback, which goes back into the 19th-century, as an independent story, we can notice the flashback with the scene of Emelia Ricoletti's suicide. She shot herself being dressed like a bride. Next day "Dead Bride's" Ghost kills her ex-husband just in front of the citizens.
Is it possible, Sherlock? Moriarty survived?
During the first meeting of Sherlock Holmes and John Watson, we figure some things about Dr. Watson from Sherlock that we were wondering after the flashback. Now we know his past and the reason of his limp for sure.
Sherlock can't remember his sister but he starts to have flashes with the Redbeard and finally all coming together: Eurus was jealous of Redbeard, and using her genius mind for such a small girl she hides him from Sherlock in the well, giving puzzle for the brother to solve if he wants to save his friend. But Sherlock is not able to figure out what does Eurus' song mean and his friend died. The psychological trauma was so serious that Sherlock's mind erased any memories of his sister and replaced the image of his friend Viktor whos nickname was Redbeard with the image of the dog to overcome painful experience.
This flashback represented unsolved case happened in the 19th century, which Sherlock builds up in his "Mind Palace" to prove himself (and viewers) that Moriarty's return is just another trick and like Emelia Reckoletti died after shooting herself, Jimm Moriarty died as well.
No, he's definitely dead. Haven't you seen a whole in his head, John? Don't be an idiot.