Mary Morstan is just a plot device. She’s neither Moriarty nor Moran, not even ‘the other one’ or Redbeard - just a shady woman with a dark past who needed a new, legitimate identity, which she aquired by marrying Doctor John Watson, a lonely, disillusioned man who’d first lost his job and purpos in livee and shortly afterwards his best friend (and secret love).
When Mary Morstan met John Watson he was vulnerable and starved for affection - he didn’t ask too many questions. And he’s charming, witty, goodlooking - a good catch.
Just imagine what would have happened to Sherlock and John if Mary hadn’t been introduced in S3? Well, how long would Mofftisson have been able to keep the dynamic going without to ‘succumb’ to Johnlock by the second episode? But if they indeed intended for at least five series, they had to keep the Sherlock-John relationship vague. They couldn’t deliver just yet because the tension between the two leads keeps the show going, especially for those who are not quite sure what’s going on but sense that there’s more to this show than just another crime drama.
It’s the Sherlock and John show. It’s about how - after over 130 years - these two will finally belong to each other. In S1 Sherlock and John meet, only to be threatened by Sherlock’s canonical arch enemy. Mofftisson didn’t knew if the series would be renewed, so they ended with a cliffhanger implying Sherlock and John willing to die for each other. What more can you give? Thisis the utmost declaration.
But the BBC ordered a second series, so they could embellish the Moriarty arch. Yet again doubting a renewal they did the three most famous stories in S2 (insane wich fullfilment even back then) and ended with Sherlock jumping, supposed to be dead. Of course we all knew he wasn’t - but John didn’t. We all knew that Sherlock would return like he did in canon. John didn’t. The hype that started after Reichenbach might have compelled the BBC to commission even more than one new series, perhaps granting permission to continue as long as possible?
So, imagine Sherlock returning in S3 without a Mary Morstan by John’s side... John would have been angry but he would have forgiven Sherlock, to solve crimes with him and move back to Baker Street. Because that’s what John Watson always does! But we already saw that in S1 and S2. Even Moriarty was dead. Sure, there are other villains in ACD canon but... been there, done that with TBB for example. The only exciting new thing would have been Johnlock. And then - what? Solve the tension within series 3 and leave so many canon stories untold, so many possibilities unexplored?
Therefore, after supervillains and death, what can you come up with to prevent your two leads to get together? Another love interest! To make it even worse, this lobe interest is introduced when one main character believes the other dead - so it’s no-ones fault really. And how tragic for the other protagonist to come back right when the other one proposes to this new love interest... this is textbook.
We got a fluffy S3. Until it wasn’t fluffy anymore because the main character was murdered. To show us that Sherlock Holmes will always die and John Watson will always grieve as long as they are not together. In S1 death only loomed; in S2 it looked like that it had happened. And even if Sherlock was still alive - John didn’t know that and the two of them were separated (which seems to have ended with both of them emotinally dying, John in his new flat, Sherlock in a dungeon in Serbia). Not good. And S3 continued this pattern: Sherlock and John were separated and Sherlock died again.
Then we got TAB, which again ended at the ominous Reichenbach Fall were Watson thinks Holmes dying. But this time he isn’t, because there’s always tow of us - they are together and not even time or history or ghosts can keep them apart. This is the first glimmer of hope - but it was set in a fairytale, removed from reality.
If Johnlock is endgame Mary has to go. A divorce is not unheard of - that’s why in TSoT it was revealed that mary is pregnant. Now the stakes are higher. But she she will go nonetheless - as in canon. With the Baby. And it’s not really important how.
Mary’s neither Moriarty nor Moran; TPTB already gave her an unusual, dark, exciting backstory - they won’t counter this very surprising plot twist with revealing her as even worse than everybody already thinks she is. That would be a bit too much. She’s not a Holmes sibling either - that would rather be very creepy imo, marrying John off to Sherlock’s sister. And whatever Redbeard is - Mary is too young to have done something traumatising when they all were but kids.
But making Mary an assassin opened up the possibility for an epic rug-pull - and Mofftisson are drama queens. Honestly, that’s all there is to Mary: she’s a dramatic distraction in the grand scheme of things. Mary is not important. She will vanish.
John might mourn her. He might grow with this loss. It might open up new character development for John Watson - and for Sherlock Holmes in dealing with this loss, comforting his friend, taking responsibility. Both might become adults finally, being able to TALK TO EACH OTHER. But that’s all Mary’s there for. She has no story arch of her own, she’s just complementing the overall narrative.
Therefore, S4 will be Mary leaving and coping with the outfall. I sincerely hope for a fifth series that will give us finally fully blown Johnlock (pun intended).
I had to get this out of my system. Thank you for listening.
