Welcome to my evil lair
Okay something is most definitely happening in this scene.
You are seeing desperation in Sherlock right now. maybe even fear. 
But what could it be?
My first thought was John, of course. 
But look at Mary. If it was John, she would be much more distressed and at his side.
And look at Mary’s face. She is sad, but not crying at the moment. And she is looking at Sherlock for the most part of it. So she knows that whoever this is, Sherlock will be affected by it.
Now who are the three people that Sherlock cares about most?
John, Lestrade, and Ms. Hudson.
John is ruled out,
And I don’t think Mary knows Lestrade all that much at this point. It could be a possibility, but by looking at this, I have a feeling this is not him. Plus, if there was someone trying to hurt Lestrade, they would not get as far as with others
Ms. Hudson is who I think is there.
Mary knows Ms. Hudson by this point im sure. I am more confident that Ms. Hudson and John stayed close then Lestrade and John did. So Mary knows her, but to close, but she knows about Sherlock and her.
If only there was a bit more of the gif, because just one second more of this could help. 
Because Sherlocks hand is going to the doorframe. He could only be putting his hand there, or grabbing on to something for support because of shock. 
In Mary’s hand there is a phone. she may have already called an ambulance.
I think that she was there before Sherlock. He comes in, then sees whatever it is there and springs forward. 
He is also looking down towards the floor, and so is Mary. 

Okay something is most definitely happening in this scene.

You are seeing desperation in Sherlock right now. maybe even fear. 

But what could it be?

My first thought was John, of course. 

But look at Mary. If it was John, she would be much more distressed and at his side.

And look at Mary’s face. She is sad, but not crying at the moment. And she is looking at Sherlock for the most part of it. So she knows that whoever this is, Sherlock will be affected by it.

Now who are the three people that Sherlock cares about most?

John, Lestrade, and Ms. Hudson.

John is ruled out,

And I don’t think Mary knows Lestrade all that much at this point. It could be a possibility, but by looking at this, I have a feeling this is not him. Plus, if there was someone trying to hurt Lestrade, they would not get as far as with others

Ms. Hudson is who I think is there.

Mary knows Ms. Hudson by this point im sure. I am more confident that Ms. Hudson and John stayed close then Lestrade and John did. So Mary knows her, but to close, but she knows about Sherlock and her.

If only there was a bit more of the gif, because just one second more of this could help. 

Because Sherlocks hand is going to the doorframe. He could only be putting his hand there, or grabbing on to something for support because of shock. 

In Mary’s hand there is a phone. she may have already called an ambulance.

I think that she was there before Sherlock. He comes in, then sees whatever it is there and springs forward. 

He is also looking down towards the floor, and so is Mary.