It all started with "The Woman".

Sherlock didn't see it, of course, but John did. Clear as day.

Irene Adler. The woman who rocked Sherlock's world.

Sherlock might have seen what Irene thought she had hidden, but Irene, she saw what Sherlock couldn't.

She saw Sherlock and John.

John and Sherlock.

Two halves of the whole.

Sherlock first felt it at Baskerville.

There was inappropriate behaviour.

"I don't have friends!"

And fear.

For the first time in ages, Sherlock felt fear.

Fear that he couldn't control everything; couldn't see everything.

Fear that he might push John away.

"I've just got one."

Make it right, Sherlock, make it right again.

Captain John Watson, Fifth Northumberland Fusiliers.

Sherlock watches John as he takes the commanding position.

His strong, upright John.

Steady, reliable John.

And he feels it.

Just the start of something.

Something new.

John doesn't notice at first. He doesn't see what Sherlock sees; feel what Sherlock feels.

What John does see is a battle.

Between Sherlock and Moriarty.

Between good and evil.

Evil wins. How can evil win?

That's not how it's supposed to go.

He watches Sherlock decline.

He watches him slip back.

"Alone protects me."

"I'm a fake."

No. No, Sherlock, you're not. I know you.

And then John sees him fall.

In that split second, he knows; believes.