Over all these years - all the crimes he's solved, the murders he's witnessed, the people he's met - nothing could compare to the enormity of this. Not Moriarty, not dismembering his entire terror network. Not even the secrets of the woman who calls herself Mary Morstan, or even Charles Augustus Magnussen's vaults could come close.
This woman is someone who cannot be controlled. She even has a hold over Mycroft, and therefore could run the country. Her intelligence could make her even more dangerous than Moriarty, should she so choose.
She has kept completely off the grid for some time now. All ten years since her eighteenth birthday, to be exact. But now she returns, and it's up to those around her to figure out why.
Sherlock's sister is visiting.