The first time he sees her is on the fifth cycle. Cycles 1-4 having ended with:
1. Shot to death by Mycroft Holmes's minions.
2. Ether-ragged by Mycroft's minions… which… did he die? It's unclear. He definitely went out of the blue and into the black and into the bedsit
3. Arrowed to death by General Shan.
4. Almost certainly didn't die, just sort of nodded off on the sofa of 221b one Sunday afternoon about a week after arriving and awakened back in the bedsit…
This time he's taking the bus, and out of nowhere Mary gets on at a stop and takes a seat directly across the aisle from him.
Did this actually happen, in what he used to laughably call his real life? Maybe. It's possible, even though he definitely doesn't remember it. There's pretty women on buses all the time (this was before he had learned his lesson on that subject), but in early 2010 he was very into Sarah and he might well not have paid a random one much attention.
He's paying attention now. Mary fills up his eyes. She's younger, just like he is, with hair down to her shoulders and a bit darker. She carries her same oversized handbag, wears a sweater he doesn't know, and a warm red-orange shawl covering her shoulders. She looks serene, and content, and beautiful, and alive…
And possibly sort of annoyed, because she's noticed him looking and given him that brief neutral eye contact that says, "Yes, I unwillingly acknowledge your existence as a human being and fellow passenger on this bus but I am not at home to amorous males today-" and he stammers, "Mary-"
She pulls out one of her earbuds and gives a faint "Someone recognizes me who I can't recall" smile and all of a sudden John realizes:
The point of all of this… is to change things.
"Yes?" Mary asks.
"Rosamund," John replies.
The gentle smile drops off her face, and it's a wonder John ever thought that this woman wasn't dangerous.
"Rosamund Mary Hunter. That's you. You've been living as Mary Morstan for a year now, you're dating David Spalding, you're a nurse, you love cats and watch every episode of Strictly Come Dancing, you like your oral sex surprisingly rough and your friend Ajay is still alive."
She's staring at him in shock, and John soldiers on, "Maybe your friend Alex, too, I don't know when he gets killed. They're being held captive by whoever you went up against in Tblisi. I don't know if you want to do anything about that. But you need to know. Maybe this way you can be safe."
The bus wheezes to a halt at the next stop, and like the coward he is, John gets out, nowhere near his destination.
He goes back to Baker Street, sleeps and wakes in his shared flat. He has adventures, and almost dies, but doesn't. Maybe it's stuck, this time.
One afternoon six months later he's climbing the seventeen steps and hears Sherlock and a woman having an animated conversation. When John opens the door he sees that Sherlock is tied in his chair but looking absolutely delighted at this turn of events.
Mary's standing in front of the fireplace, carrying a silenced revolver.
"Ah, history man, you're here, good!" she says, smiling, "You know a surprising number of true facts. I thought we could have a chat about how."
Notes: This is hopefully going to be only the first installment in a multi-writer, multi-chaptered AU. Said AU is open to all, though unfortunately I can't post the link in here. However if you care to visit at AO3 (I'm the same screen name) you may click on this story and see the collection, submit and fill prompts, and so on.