The first time they meet, he's a young MI6 agent on a dangerous mission.
He's just made a stupid mistake – the concern for his little brother causing him to lose his focus for the briefest of moments – and now his enemy has got the better of him. He clenches his jaw, steels himself for a bullet that never comes.
His opponent slumps to the ground without a sound, and he whips his head around to find her still wielding her gun.
"You're slow," she sneers, and he holds her gaze for a silent moment.
She can't be more than nineteen, but there's a steely determination in her eyes that he's seldom happened to see. CIA agent, most likely; they're not exactly on the same side, but they're not on opposite sides either.
He acknowledges her with a slight nod, watches her as she stalks away.
xxx
It's twenty years later when he hears from her again.
I know you're watching John, she texts him. Just wanted to let you know that I'll take care of him from now on.
He traces the number back to a part-time nurse that goes by the name of Mary Morstan. Except that Mary Morstan was stillborn in October 1972, and it doesn't take Mycroft Holmes to guess what's going on there.
He's under my protection, he replies at last. You'd better watch your step.
Duly noted, sir, she texts back, irony seeping through the lines of the short message.
He shakes his head, wondering whether Sherlock would approve. However, his little brother is far away on a solitary mission, and John Watson needs someone else than Mycroft to look after him.
Someone as dangerous as his former flatmate – or possibly even more dangerous, as a matter of fact.
That's what the good doctor likes after all.
xxx
It's probably no wonder that Sherlock and Mary like each other. They're more similar than they care to admit, and they're both in love with the former army doctor.
He still thinks that her skills are wasted on the role she's playing now, and yet John Watson must be a very special man for his little brother to be so fond of him. Sherlock delivering a best man's speech is tempting indeed, but in the end he refrains from attending the wedding reception.
What Mycroft hasn't foreseen is the incident at Magnussen's office. Stupid little boy, how has he not been able to see right through Mary Watson's lies?
Now Sherlock is lying in a hospital bed, and he's sorely tempted to tell John the truth about his wife. However, he's well aware that that would break the doctor for good; his brother is never going to forgive him if he does anything of the kind.
His deductions confirm that she never really intended to kill Sherlock – mostly for John's sake – and that any other trained agent would have probably done the same. In the end he meets her gaze when John can't see, makes sure she understands exactly what he thinks of this.
She doesn't blink, simply wraps her hand around her husband's shoulder.